Javascript——概述 && 继承 && 复用 && 私有成员 && 构造函数

原文链接：A re-introduction to JavaScript (JS tutorial)

Why a re-introduction? Because JavaScript is notorious for being the world's most misunderstood programming language.

It is often derided as being a toy, but beneath its layer of deceptive simplicity, powerful language features await. JavaScript is now used by an incredible number of high-profile applications, showing that deeper knowledge of this technology is an important skill for any web or mobile developer.

Several months after that, Netscape submitted JavaScript to Ecma International, a European standards organization, which resulted in the first edition of the ECMAScript standard that year.

Because it is more familiar, we will refer to ECMAScript as "JavaScript" from this point on.

Unlike most programming languages, the JavaScript language has no concept of input or output. It is designed to run as a scripting language in a host environment, and it is up to the host environment to provide mechanisms for communicating with the outside world.

The most common host environment is the browser, but JavaScript interpreters can also be found in a huge list of other places, including Adobe Acrobat, Adobe Photoshop, SVG images, Yahoo's Widget engine, server-side environments such as Node.js, NoSQL databases like the open source Apache CouchDB, embedded computers, complete desktop environments like GNOME(one of the most popular GUIs for GNU/Linux operating systems), and others

Overview

JavaScript is a multi-paradigm, dynamic language with types and operators, standard built-in objects, and methods.

Its syntax is based on the Java and C languages — many structures from those languages apply to JavaScript as well.

• JavaScript supports object-oriented programming with object prototypes, instead of classes (see more about prototypical inheritance and ES2015 classes).
• JavaScript also supports functional programming — because they are objects, functions may be stored in variables and passed around like any other object.

Let's start off by looking at the building blocks of any language: the types. JavaScript programs manipulate values, and those values all belong to a type. JavaScript's types are:

• Number
• String
• Boolean
• Function
• Object
• Symbol (new in ES2015)

• Object
  • Function
  • Array
  • Date
  • RegExp
• null
• undefined
• Error

... oh, and undefined and null, which are ... slightly odd.

And Array, which is a special kind of object.

And Date and RegExp, which are objects that you get for free.

And to be technically accurate, functions are just a special type of object.

And there are some built-in Error types as well.

Numbers

Numbers in JavaScript are "double-precision 64-bit format IEEE 754 values", according to the spec.

This has some interesting consequences. There's no such thing as an integer in JavaScript, so you have to be a little careful with your arithmetic if you're used to math in C or Java.

Also, watch out for stuff like:

0.1 + 0.2 == 0.30000000000000004;

In practice, integer values are treated as 32-bit ints, and some implementations even store it that way until they are asked to perform an instruction that's valid on a Number but not on a 32-bit integer. This can be important for bit-wise operations（逐位运算）.

The standard arithmetic operators are supported, including addition, subtraction, modulus (or remainder) arithmetic, and so forth.

There's also a built-in object that we did not mention earlier called Math that provides advanced mathematical functions and constants:

Math.sin(3.5);
var circumference = 2 * Math.PI * r;

You can convert a string to an integer using the built-in parseInt() function. T

his takes the base for the conversion as an optional second argument, which you should always provide:

parseInt('123', 10); // 123
parseInt('010', 10); // 10

In older browsers, strings beginning with a "0" are assumed to be in octal (radix 8), but this hasn't been the case since 2013 or so. Unless you're certain of your string format, you can get surprising results on those older browsers:

parseInt('010');  //  8
parseInt('0x10'); // 16

Here, we see the parseInt() function treat the first string as octal due to the leading 0, and the second string as hexadecimal due to the leading "0x". The hexadecimal notation is still in place; only octal has been removed.

If you want to convert a binary number to an integer, just change the base:

parseInt('11', 2); // 3

Similarly, you can parse floating point numbers using the built-in parseFloat() function. Unlike its parseInt() cousin, parseFloat() always uses base 10.

You can also use the unary + operator to convert values to numbers:

+ '42';   // 42
+ '010';  // 10
+ '0x10'; // 16

A special value called NaN (short for "Not a Number") is returned if the string is non-numeric:

parseInt('hello', 10); // NaN

NaN is toxic: if you provide it as an operand to any mathematical operation, the result will also be NaN:

NaN + 5; // NaN

You can test for NaN using the built-in isNaN() function:

isNaN(NaN); // true

JavaScript also has the special values Infinity and -Infinity:

 1 / 0; //  Infinity
-1 / 0; // -Infinity

You can test for Infinity, -Infinity and NaN values using the built-in isFinite()function:

isFinite(1 / 0); // false
isFinite(-Infinity); // false
isFinite(NaN); // false

The parseInt() and parseFloat() functions parse a string until they reach a character that isn't valid for the specified number format, then return the number parsed up to that point.

However the "+" operator simply converts the string to NaN if there is an invalid character contained within it.

Just try parsing the string "10.2abc" with each method by yourself in the console and you'll understand the differences better.

Strings

Strings in JavaScript are sequences of Unicode characters. This should be welcome news to anyone who has had to deal with internationalization. More accurately, they are sequences of UTF-16 code units; each code unit is represented by a 16-bit number. Each Unicode character is represented by either 1 or 2 code units.

If you want to represent a single character, you just use a string consisting of that single character.

To find the length of a string (in code units), access its length property:

'hello'.length; // 5

There's our first brush with JavaScript objects! Did we mention that you can use strings like objects too? They have methods as well that allow you to manipulate the string and access information about the string:

'hello'.charAt(0); // "h"
'hello, world'.replace('world', 'mars'); // "hello, mars"
'hello'.toUpperCase(); // "HELLO"

Other types

JavaScript distinguishes between null, which is a value that indicates a deliberate non-value (and is only accessible through the null keyword), and undefined, which is a value of type undefined that indicates an uninitialized variable — that is, a value hasn't even been assigned yet.

We'll talk about variables later, but in JavaScript it is possible to declare a variable without assigning a value to it. If you do this, the variable's type is undefined. undefined is actually a constant.

JavaScript has a boolean type, with possible values true and false (both of which are keywords.) Any value can be converted to a boolean according to the following rules:

1. false, 0, empty strings (""), NaN, null, and undefined all become false.
2. All other values become true.

You can perform this conversion explicitly using the Boolean() function:

Boolean('');  // false
Boolean(234); // true

However, this is rarely necessary, as JavaScript will silently perform this conversion when it expects a boolean, such as in an if statement (see below).

For this reason, we sometimes speak simply of "true values" and "false values," meaning values that become true and false, respectively, when converted to booleans. Alternatively, such values can be called "truthy" and "falsy", respectively.

Boolean operations such as && (logical and), || (logical or), and ! (logical not) are supported; see below.

Variables

New variables in JavaScript are declared using one of three keywords: let, const, or var.

let allows you to declare block-level variables. The declared variable is available from the block it is enclosed in.

let a;
let name = 'Simon';

The following is an example of scope with a variable declared with let:

// myLetVariable is *not* visible out here

for (let myLetVariable = 0; myLetVariable < 5; myLetVariable++) {
  // myLetVariable is only visible in here
}

// myLetVariable is *not* visible out here

 

const allows you to declare variables whose values are never intended to change. The variable is available from the block it is declared in.

const Pi = 3.14; // variable Pi is set
Pi = 1; // will throw an error because you cannot change a constant variable.

var is the most common declarative keyword. It does not have the restrictions that the other two keywords have. This is because it was traditionally the only way to declare a variable in JavaScript. A variable declared with the var keyword is available from the function it is declared in.

var a;
var name = 'Simon';

An example of scope with a variable declared with var:

// myVarVariable *is* visible out here

for (var myVarVariable = 0; myVarVariable < 5; myVarVariable++) {
  // myVarVariable is visible to the whole function
} 

// myVarVariable *is* visible out here

If you declare a variable without assigning any value to it, its type is undefined.

An important difference between JavaScript and other languages like Java is that in JavaScript, blocks do not have scope; only functions have a scope. So if a variable is defined using var in a compound statement (for example inside an if control structure), it will be visible to the entire function. However, starting with ECMAScript 2015, let and const declarations allow you to create block-scoped variables.

Operators

JavaScript's numeric operators are +, -, *, / and % which is the remainder operator (which is the same as modulo.) Values are assigned using =, and there are also compound assignment statements such as += and -=. These extend out to x = x operator y.

x += 5;
x = x + 5;

You can use ++ and -- to increment and decrement respectively. These can be used as a prefix or postfix operators.

The + operator also does string concatenation:

'hello' + ' world'; // "hello world"

If you add a string to a number (or other value) everything is converted into a string first. This might trip you up:

'3' + 4 + 5;  // "345"
 3 + 4 + '5'; // "75"

Adding an empty string to something is a useful way of converting it to a string itself.

Comparisons in JavaScript can be made using <, >, <= and >=. These work for both strings and numbers. Equality is a little less straightforward. The double-equals operator performs type coercion if you give it different types, with sometimes interesting results:

123 == '123'; // true
1 == true; // true

To avoid type coercion, use the triple-equals operator:

123 === '123'; // false
1 === true;    // false

There are also != and !== operators.

JavaScript also has bitwise operations. If you want to use them, they're there.

Control structures

JavaScript has a similar set of control structures to other languages in the C family.

Conditional statements are supported by if and else; you can chain them together if you like:

var name = 'kittens';
if (name == 'puppies') {
  name += ' woof';
} else if (name == 'kittens') {
  name += ' meow';
} else {
  name += '!';
}
name == 'kittens meow';

JavaScript has while loops and do-while loops. The first is good for basic looping; the second for loops where you wish to ensure that the body of the loop is executed at least once:

while (true) {
  // an infinite loop!
}

var input;
do {
  input = get_input();
} while (inputIsNotValid(input));

JavaScript's for loop is the same as that in C and Java: it lets you provide the control information for your loop on a single line.

for (var i = 0; i < 5; i++) {
  // Will execute 5 times
}

JavaScript also contains two other prominent for loops: for...of

for (let value of array) {
  // do something with value
}

and for...in:

for (let property in object) {
  // do something with object property
}

The && and || operators use short-circuit logic, which means whether they will execute their second operand is dependent on the first. This is useful for checking for null objects before accessing their attributes:

var name = o && o.getName();

Or for caching values (when falsy values are invalid):

var name = cachedName || (cachedName = getName());

JavaScript has a ternary operator for conditional expressions:

var allowed = (age > 18) ? 'yes' : 'no';

The switch statement can be used for multiple branches based on a number or string:

switch (action) {
  case 'draw':
    drawIt();
    break;
  case 'eat':
    eatIt();
    break;
  default:
    doNothing();
}

If you don't add a break statement, execution will "fall through" to the next level. This is very rarely what you want — in fact it's worth specifically labeling deliberate fallthrough with a comment if you really meant it to aid debugging:

switch (a) {
  case 1: // fallthrough
  case 2:
    eatIt();
    break;
  default:
    doNothing();
}

The default clause is optional. You can have expressions in both the switch part and the cases if you like; comparisons take place between the two using the === operator:

switch (1 + 3) {
  case 2 + 2:
    yay();
    break;
  default:
    neverhappens();
}

Objects

JavaScript objects can be thought of as simple collections of name-value pairs. As such, they are similar to:

• Dictionaries in Python.
• Hashes in Perl and Ruby.
• Hash tables in C and C++.
• HashMaps in Java.
• Associative arrays in PHP.

The fact that this data structure is so widely used is a testament to its versatility. Since everything (bare core types) in JavaScript is an object, any JavaScript program naturally involves a great deal of hash table lookups. It's a good thing they're so fast!

The "name" part is a JavaScript string, while the value can be any JavaScript value — including more objects. This allows you to build data structures of arbitrary complexity.

There are two basic ways to create an empty object:

var obj = new Object();

And:

var obj = {};

These are semantically equivalent; the second is called object literal syntax and is more convenient. This syntax is also the core of JSON format and should be preferred at all times.

Object literal syntax can be used to initialize an object in its entirety:

var obj = {
  name: 'Carrot',
  for: 'Max', // 'for' is a reserved word, use '_for' instead.
  details: {
    color: 'orange',
    size: 12
  }
};

Attribute access can be chained together:

obj.details.color; // orange
obj['details']['size']; // 12

The following example creates an object prototype(Person) and an instance of that prototype(you).

function Person(name, age) {
  this.name = name;
  this.age = age;
}

// Define an object
var you = new Person('You', 24);
// We are creating a new person named "You" aged 24.

Once created, an object's properties can again be accessed in one of two ways:

// dot notation
obj.name = 'Simon';
var name = obj.name;

And...

// bracket notation
obj['name'] = 'Simon';
var name = obj['name'];
// can use a variable to define a key
var user = prompt('what is your key?')
obj[user] = prompt('what is its value?')

These are also semantically equivalent. The second method has the advantage that the name of the property is provided as a string, which means it can be calculated at run-time. However, using this method prevents some JavaScript engine and minifier optimizations being applied. It can also be used to set and get properties with names that are reserved words:

obj.for = 'Simon'; // Syntax error, because 'for' is a reserved word
obj['for'] = 'Simon'; // works fine

Starting in ECMAScript 5, reserved words may be used as object property names "in the buff". This means that they don't need to be "clothed" in quotes when defining object literals. See the ES5 Spec.

For more on objects and prototypes see Object.prototype. For an explanation of object prototypes and the object prototype chains see Inheritance and the prototype chain.

Starting in ECMAScript 2015, object keys can be defined by the variable using bracket notation upon being created. {[phoneType]: 12345} is possible instead of just var userPhone = {}; userPhone[phoneType] = 12345.

Arrays

Arrays in JavaScript are actually a special type of object. They work very much like regular objects (numerical properties can naturally be accessed only using [] syntax) but they have one magic property called 'length'. This is always one more than the highest index in the array.

One way of creating arrays is as follows:

var a = new Array();
a[0] = 'dog';
a[1] = 'cat';
a[2] = 'hen';
a.length; // 3

A more convenient notation is to use an array literal:

var a = ['dog', 'cat', 'hen'];
a.length; // 3

Note that array.length isn't necessarily the number of items in the array. Consider the following:

var a = ['dog', 'cat', 'hen'];
a[100] = 'fox';
a.length; // 101

Remember — the length of the array is one more than the highest index.

If you query a non-existent array index, you'll get a value of undefined in return:

typeof a[90]; // undefined

If you take the above about [] and length into account, you can iterate over an array using the following for loop:

for (var i = 0; i < a.length; i++) {
  // Do something with a[i]
}

ES2015 introduced the more concise for...of loop for iterable objects such as arrays:

for (const currentValue of a) {
  // Do something with currentValue
}

You could also iterate over an array using a for...in loop, however this does not iterate over the array elements, but the array indices. Furthermore, if someone added new properties to Array.prototype, they would also be iterated over by such a loop. Therefore this loop type is not recommended for arrays.

Another way of iterating over an array that was added with ECMAScript 5 is forEach():

['dog', 'cat', 'hen'].forEach(function(currentValue, index, array) {
  // Do something with currentValue or array[index]
});

If you want to append an item to an array simply do it like this:

a.push(item);

Arrays come with a number of methods. See also the full documentation for array methods.

Method name	Description
a.toString()	Returns a string with the toString() of each element separated by commas.
a.toLocaleString()	Returns a string with the toLocaleString() of each element separated by commas.
a.concat(item1[, item2[, ...[, itemN]]])	Returns a new array with the items added on to it.
a.join(sep)	Converts the array to a string — with values delimited by the sep param
a.pop()	Removes and returns the last item.
a.push(item1, ..., itemN)	Appends items to the end of the array.
a.reverse()	Reverses the array.
a.shift()	Removes and returns the first item.
a.slice(start[, end])	Returns a sub-array.
a.sort([cmpfn])	Takes an optional comparison function.
a.splice(start, delcount[, item1[, ...[, itemN]]])	Lets you modify an array by deleting a section and replacing it with more items.
a.unshift(item1[, item2[, ...[, itemN]]])	Prepends items to the start of the array.

Functions

Along with objects, functions are the core component in understanding JavaScript.

The most basic function couldn't be much simpler:

function add(x, y) {
  var total = x + y;
  return total;
}

This demonstrates a basic function.

• A JavaScript function can take 0 or more named parameters.
• The function body can contain as many statements as you like and can declare its own variables which are local to that function.
• The return statement can be used to return a value at any time, terminating the function. If no return statement is used (or an empty return with no value), JavaScript returns undefined.

The named parameters turn out to be more like guidelines than anything else. You can call a function without passing the parameters it expects, in which case they will be set to undefined.

add(); // NaN
// You can't perform addition on undefined

You can also pass in more arguments than the function is expecting:

add(2, 3, 4); // 5
// added the first two; 4 was ignored

That may seem a little silly, but functions have access to an additional variable inside their body called arguments, which is an array-like object holding all of the values passed to the function. Let's re-write the add function to take as many values as we want:

function add() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum;
}

add(2, 3, 4, 5); // 14

That's really not any more useful than writing 2 + 3 + 4 + 5 though. Let's create an averaging function:

function avg() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum / arguments.length;
}

avg(2, 3, 4, 5); // 3.5

This is pretty useful, but it does seem a little verbose. To reduce this code a bit more we can look at substituting the use of the arguments array through Rest parameter syntax. In this way, we can pass in any number of arguments into the function while keeping our code minimal. The rest parameter operator is used in function parameter lists with the format: ...variable and it will include within that variable the entire list of uncaptured arguments that the function was called with. We will also replace the for loop with a for...of loop to return the values within our variable.

function avg(...args) {
  var sum = 0;
  for (let value of args) {
    sum += value;
  }
  return sum / args.length;
}

avg(2, 3, 4, 5); // 3.5

In the above code, the variable args holds all the values that were passed into the function.

It is important to note that wherever the rest parameter operator is placed in a function declaration it will store all arguments after its declaration, but not before. i.e. functionavg(firstValue, ...args) will store the first value passed into the function in the firstValue variable and the remaining arguments in args. That's another useful language feature but it does lead us to a new problem.

The avg() function takes a comma-separated list of arguments — but what if you want to find the average of an array? You could just rewrite the function as follows:

function avgArray(arr) {
  var sum = 0;
  for (var i = 0, j = arr.length; i < j; i++) {
    sum += arr[i];
  }
  return sum / arr.length;
}

avgArray([2, 3, 4, 5]); // 3.5

But it would be nice to be able to reuse the function that we've already created. Luckily, JavaScript lets you call a function with an arbitrary array of arguments, using the apply()method of any function object.

avg.apply(null, [2, 3, 4, 5]); // 3.5

The second argument to apply() is the array to use as arguments; the first will be discussed later on. This emphasizes the fact that functions are objects too.

You can achieve the same result using the spread operator in the function call.

For instance: avg(...numbers)

JavaScript lets you create anonymous functions.

var avg = function() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum / arguments.length;
};

This is semantically equivalent to the function avg() form. It's extremely powerful, as it lets you put a full function definition anywhere that you would normally put an expression. This enables all sorts of clever tricks. Here's a way of "hiding" some local variables — like block scope in C:

var a = 1;
var b = 2;

(function() {
  var b = 3;
  a += b;
})();

a; // 4
b; // 2

JavaScript allows you to call functions recursively. This is particularly useful for dealing with tree structures, such as those found in the browser DOM.

function countChars(elm) {
  if (elm.nodeType == 3) { // TEXT_NODE
    return elm.nodeValue.length;
  }
  var count = 0;
  for (var i = 0, child; child = elm.childNodes[i]; i++) {
    count += countChars(child);
  }
  return count;
}

This highlights a potential problem with anonymous functions: how do you call them recursively if they don't have a name? JavaScript lets you name function expressions for this. You can use named IIFEs (Immediately Invoked Function Expressions) as shown below:

var charsInBody = (function counter(elm) {
  if (elm.nodeType == 3) { // TEXT_NODE
    return elm.nodeValue.length;
  }
  var count = 0;
  for (var i = 0, child; child = elm.childNodes[i]; i++) {
    count += counter(child);
  }
  return count;
})(document.body);

The name provided to a function expression as above is only available to the function's own scope. This allows more optimizations to be done by the engine and results in more readable code. The name also shows up in the debugger and some stack traces, which can save you time when debugging.

Note that JavaScript functions are themselves objects — like everything else in JavaScript — and you can add or change properties on them just like we've seen earlier in the Objects section.

Custom objects

For a more detailed discussion of object-oriented programming in JavaScript, see Introduction to Object-Oriented JavaScript.

In classic Object Oriented Programming, objects are collections of data and methods that operate on that data. JavaScript is a prototype-based language that contains no class statement, as you'd find in C++ or Java (this is sometimes confusing for programmers accustomed to languages with a class statement). Instead, JavaScript uses functions as classes. Let's consider a person object with first and last name fields. There are two ways in which the name might be displayed: as "first last" or as "last, first". Using the functions and objects that we've discussed previously, we could display the data like this:

function makePerson(first, last) {
  return {
    first: first,
    last: last
  };
}
function personFullName(person) {
  return person.first + ' ' + person.last;
}
function personFullNameReversed(person) {
  return person.last + ', ' + person.first;
}

var s = makePerson('Simon', 'Willison');
personFullName(s); // "Simon Willison"
personFullNameReversed(s); // "Willison, Simon"

This works, but it's pretty ugly.You end up with dozens of functions in your global namespace. What we really need is a way to attach a function to an object. Since functions are objects, this is easy:

function makePerson(first, last) {
  return {
    first: first,
    last: last,
    fullName: function() {
      return this.first + ' ' + this.last;
    },
    fullNameReversed: function() {
      return this.last + ', ' + this.first;
    }
  };
}

var s = makePerson('Simon', 'Willison');
s.fullName(); // "Simon Willison"
s.fullNameReversed(); // "Willison, Simon"

Note on the this keyword. Used inside a function, this refers to the current object. What that actually means is specified by the way in which you called that function. If you called it using dot notation or bracket notation on an object, that object becomes this. If dot notation wasn't used for the call, this refers to the global object.

Note that this is a frequent cause of mistakes. For example:

var s = makePerson('Simon', 'Willison');
var fullName = s.fullName;
fullName(); // undefined undefined

When we call fullName() alone, without using s.fullName(), this is bound to the global object. Since there are no global variables called first or last we get undefined for each one.

We can take advantage of the this keyword to improve our makePerson function:

function Person(first, last) {
  this.first = first;
  this.last = last;
  this.fullName = function() {
    return this.first + ' ' + this.last;
  };
  this.fullNameReversed = function() {
    return this.last + ', ' + this.first;
  };
}
var s = new Person('Simon', 'Willison');

We have introduced another keyword: new. new is strongly related to this. It creates a brand new empty object, and then calls the function specified, with this set to that new object. Notice though that the function specified with this does not return a value but merely modifies the this object. It's new that returns the this object to the calling site. Functions that are designed to be called by new are called constructor functions. Common practice is to capitalize these functions as a reminder to call them with new.

The improved function still has the same pitfall with calling fullName() alone.

Our person objects are getting better, but there are still some ugly edges to them. Every time we create a person object we are creating two brand new function objects within it — wouldn't it be better if this code was shared?

function personFullName() {
  return this.first + ' ' + this.last;
}
function personFullNameReversed() {
  return this.last + ', ' + this.first;
}
function Person(first, last) {
  this.first = first;
  this.last = last;
  this.fullName = personFullName;
  this.fullNameReversed = personFullNameReversed;
}

That's better: we are creating the method functions only once, and assigning references to them inside the constructor. Can we do any better than that? The answer is yes:

function Person(first, last) {
  this.first = first;
  this.last = last;
}
Person.prototype.fullName = function() {
  return this.first + ' ' + this.last;
};
Person.prototype.fullNameReversed = function() {
  return this.last + ', ' + this.first;
};

Person.prototype is an object shared by all instances of Person. It forms part of a lookup chain (that has a special name, "prototype chain"): any time you attempt to access a property of Person that isn't set, JavaScript will check Person.prototype to see if that property exists there instead. As a result, anything assigned to Person.prototype becomes available to all instances of that constructor via the this object.

This is an incredibly powerful tool. JavaScript lets you modify something's prototype at any time in your program, which means you can add extra methods to existing objects at runtime:

var s = new Person('Simon', 'Willison');
s.firstNameCaps(); // TypeError on line 1: s.firstNameCaps is not a function

Person.prototype.firstNameCaps = function() {
  return this.first.toUpperCase();
};
s.firstNameCaps(); // "SIMON"

Interestingly, you can also add things to the prototype of built-in JavaScript objects. Let's add a method to String that returns that string in reverse:

var s = 'Simon';
s.reversed(); // TypeError on line 1: s.reversed is not a function

String.prototype.reversed = function() {
  var r = '';
  for (var i = this.length - 1; i >= 0; i--) {
    r += this[i];
  }
  return r;
};

s.reversed(); // nomiS

Our new method even works on string literals!

'This can now be reversed'.reversed(); // desrever eb won nac sihT

As mentioned before, the prototype forms part of a chain. The root of that chain is Object.prototype, whose methods include toString() — it is this method that is called when you try to represent an object as a string. This is useful for debugging our Personobjects:

var s = new Person('Simon', 'Willison');
s.toString(); // [object Object]

Person.prototype.toString = function() {
  return '<Person: ' + this.fullName() + '>';
}

s.toString(); // "<Person: Simon Willison>"

Remember how avg.apply() had a null first argument? We can revisit that now. The first argument to apply() is the object that should be treated as 'this'. For example, here's a trivial implementation of new:

function trivialNew(constructor, ...args) {
  var o = {}; // Create an object
  constructor.apply(o, args);
  return o;
}

This isn't an exact replica of new as it doesn't set up the prototype chain (it would be difficult to illustrate). This is not something you use very often, but it's useful to know about. In this snippet, ...args (including the ellipsis) is called the "rest arguments" — as the name implies, this contains the rest of the arguments.

Calling

var bill = trivialNew(Person, 'William', 'Orange');

is therefore almost equivalent to

var bill = new Person('William', 'Orange');

apply() has a sister function named call, which again lets you set this but takes an expanded argument list as opposed to an array.

apply() has a sister function named call, which again lets you set this but takes an expanded argument list as opposed to an array.

function lastNameCaps() {
  return this.last.toUpperCase();
}
var s = new Person('Simon', 'Willison');
lastNameCaps.call(s);
// Is the same as:
s.lastNameCaps = lastNameCaps;
s.lastNameCaps(); // WILLISON

Inner functions

JavaScript function declarations are allowed inside other functions. We've seen this once before, with an earlier makePerson() function. An important detail of nested functions in JavaScript is that they can access variables in their parent function's scope:

function parentFunc() {
  var a = 1;

  function nestedFunc() {
    var b = 4; // parentFunc can't use this
    return a + b;
  }
  return nestedFunc(); // 5
}

This provides a great deal of utility in writing more maintainable code. If a called function relies on one or two other functions that are not useful to any other part of your code, you can nest those utility functions inside it. This keeps the number of functions that are in the global scope down, which is always a good thing.

This is also a great counter to the lure of global variables. When writing complex code it is often tempting to use global variables to share values between multiple functions — which leads to code that is hard to maintain. Nested functions can share variables in their parent, so you can use that mechanism to couple functions together when it makes sense without polluting your global namespace — "local globals" if you like. This technique should be used with caution, but it's a useful ability to have.

Closures

This leads us to one of the most powerful abstractions that JavaScript has to offer — but also the most potentially confusing. What does this do?

function makeAdder(a) {
  return function(b) {
    return a + b;
  };
}
var add5 = makeAdder(5);
var add20 = makeAdder(20);
add5(6); // ?
add20(7); // ?

The name of the makeAdder() function should give it away: it creates new 'adder' functions, each of which, when called with one argument, adds it to the argument that it was created with.

What's happening here is pretty much the same as was happening with the inner functions earlier on: a function defined inside another function has access to the outer function's variables. The only difference here is that the outer function has returned, and hence common sense would seem to dictate that its local variables no longer exist. But they do still exist — otherwise, the adder functions would be unable to work. What's more, there are two different "copies" of makeAdder()'s local variables — one in which a is 5 and the other one where a is 20. So the result of that function calls is as follows:

add5(6); // returns 11
add20(7); // returns 27

Here's what's actually happening. Whenever JavaScript executes a function, a 'scope' object is created to hold the local variables created within that function. It is initialized with any variables passed in as function parameters. This is similar to the global object that all global variables and functions live in, but with a couple of important differences: firstly, a brand new scope object is created every time a function starts executing, and secondly, unlike the global object (which is accessible as this and in browsers as window) these scope objects cannot be directly accessed from your JavaScript code. There is no mechanism for iterating over the properties of the current scope object, for example.

So when makeAdder() is called, a scope object is created with one property: a, which is the argument passed to the makeAdder() function. makeAdder() then returns a newly created function. Normally JavaScript's garbage collector would clean up the scope object created for makeAdder() at this point, but the returned function maintains a reference back to that scope object. As a result, the scope object will not be garbage-collected until there are no more references to the function object that makeAdder() returned.

Scope objects form a chain called the scope chain, similar to the prototype chain used by JavaScript's object system.

A closure is the combination of a function and the scope object in which it was created. Closures let you save state — as such, they can often be used in place of objects. You can find several excellent introductions to closures.

JavaScript:The World's Most Misunderstood Programming Language

JavaScript, aka Mocha, aka LiveScript, aka JScript, aka ECMAScript, is one of the world's most popular programming languages.

Despite its popularity, few know that JavaScript is a very nice dynamic object-oriented general-purpose（多用途） programming language.

The Name

Java is interpreted Java. JavaScript is a different language.

JavaScript has a syntactic similarity to Java, much as Java has to C.But it is no more a subset of Java than Java is a subset of C

JavaScript was not developed at Sun Microsystems, the home of Java. JavaScript was developed at Netscape. It was originally called LiveScript, but that name wasn't confusing enough.

The -Script suffix suggests that it is not a real programming language, that a scripting language is less than a programming language. But it is really a matter of specialization. Compared to C, JavaScript trades performance for expressive power and dynamism.

Lisp in C's Clothing

This is misleading because JavaScript has more in common with functional languages like Lisp or Scheme than with C or Java.

It has arrays instead of lists and objects instead of property lists.

Functions are first class. It has closures.

You get lambdas without having to balance all those parens.

Typecasting

JavaScript was designed to run in Netscape Navigator. Its success there led to it becoming standard equipment in virtually all web browsers. This has resulted in typecasting.

JavaScript is the George Reevesof programming languages.

JavaScript is well suited to a large class of non-Web-related applications

Moving Target

The first versions of JavaScript were quite weak. They lacked exception handling, inner functions, and inheritance. In its present form, it is now a complete object-oriented programming language. But many opinions of the language are based on its immature forms.

The ECMA committee that has stewardship over the language is developing extensions which, while well intentioned, will aggravate one of the language's biggest problems: There are already too many versions. This creates confusion.

Design Errors

No programming language is perfect. JavaScript has its share of design errors,

• such as the overloading of + to mean both addition and concatenation with type coercion, and the error-prone with statement should be avoided.
• The reserved word policies are much too strict.
• Semicolon insertion was a huge mistake, as was the notation for literal regular expressions.

These mistakes have led to programming errors, and called the design of the language as a whole into question. Fortunately, many of these problems can be mitigated with a good lint program.

The design of the language on the whole is quite sound. Surprisingly, the ECMAScript committee does not appear to be interested in correcting these problems. Perhaps they are more interested in making new ones.

Lousy Implementations

Some of the earlier implementations of JavaScript were quite buggy. This reflected badly on the language. Compounding that, those implementations were embedded in horribly buggy web browsers.

Bad Books

Nearly all of the books about JavaScript are quite awful. They contain errors, poor examples, and promote bad practices. Important features of the language are often explained poorly, or left out entirely. I have reviewed dozens of JavaScript books, and I can only recommend one: JavaScript: The Definitive Guide (5th Edition) by David Flanagan. (Attention authors: If you have written a good one, please send me a review copy.)

Substandard Standard

The official specification for the language is published by ECMA. The specification is of extremely poor quality. It is difficult to read and very difficult to understand. This has been a contributor to the Bad Book problem because authors have been unable to use the standard document to improve their own understanding of the language. ECMA and the TC39 committee should be deeply embarrassed.

Amateurs

Most of the people writing in JavaScript are not programmers. They lack the training and discipline to write good programs. JavaScript has so much expressive power that they are able to do useful things in it, anyway. This has given JavaScript a reputation of being strictly for the amateurs, that it is not suitable for professional programming. This is simply not the case.

Object-Oriented

Is JavaScript object-oriented? It has objects which can contain data and methods that act upon that data. Objects can contain other objects. It does not have classes, but it does have constructors which do what classes do, including acting as containers for class variables and methods. It does not have class-oriented inheritance, but it does have prototype-oriented inheritance.

The two main ways of building up object systems are by inheritance (is-a) and by aggregation (has-a). JavaScript does both, but its dynamic nature allows it to excel at aggregation.

Some argue that JavaScript is not truly object oriented because it does not provide information hiding. That is, objects cannot have private variables and private methods: All members are public.

But it turns out that JavaScript objects can have private variables and private methods. (Click here now to find out how.) Of course, few understand this because JavaScript is the world's most misunderstood programming language.

Some argue that JavaScript is not truly object oriented because it does not provide inheritance. But it turns out that JavaScript supports not only classical inheritance, but other code reuse patterns as well.

Private Members in JavaScript

Some believe that it lacks the property of information hiding because objects cannot have private instance variables and methods. But this is a misunderstanding. JavaScript objects can have private members.

Objects

JavaScript is fundamentally about objects. Arrays are objects. Functions are objects. Objects are objects.

So what are objects? Objects are collections of name-value pairs. The names are strings, and the values are strings, numbers, booleans, and objects (including arrays and functions)

Objects are usually implemented as hashtables（哈希表） so values can be retrieved quickly.

If a value is a function, we can consider it a method. When a method of an object is invoked, the thisvariable is set to the object. The method can then access the instance variables through the this variable.

Objects can be produced by constructors, which are functions which initialize objects.

Constructors provide the features that classes provide in other languages, including static variables and methods.

Public

The members of an object are all public members. Any function can access, modify, or delete those members, or add new members. There are two main ways of putting members in a new object:

In the constructor

This technique is usually used to initialize public instance variables. The constructor's this variable is used to add members to the object.

function Container(param) {
    this.member = param;
}

So, if we construct a new object

var myContainer = new Container('abc');

then myContainer.member contains 'abc'.

In the prototype

This technique is usually used to add public methods. When a member is sought and it isn't found in the object itself, then it is taken from the object's constructor's prototype member. The prototype mechanism is used for inheritance. It also conserves memory. To add a method to all objects made by a constructor, add a function to the constructor's prototype:

Container.prototype.stamp = function (string) {
    return this.member + string;
}

So, we can invoke the method

myContainer.stamp('def')

which produces 'abcdef'.

Private

Private members are made by the constructor. Ordinary vars and parameters of the constructor become the private members.

function Container(param) {
    this.member = param;
    var secret = 3;
    var that = this;
}

This constructor makes three private instance variables: param, secret, and that. They are attached to the object, but they are not accessible to the outside, nor are they accessible to the object's own public methods. They are accessible to private methods. Private methods are inner functions of the constructor.

function Container(param) {

    function dec() {
        if (secret > 0) {
            secret -= 1;
            return true;
        } else {
            return false;
        }
    }

    this.member = param;
    var secret = 3;
    var that = this;
}

The private method dec examines the secret instance variable. If it is greater than zero, it decrements secretand returns true. Otherwise it returns false. It can be used to make this object limited to three uses.

By convention, we make a private that variable. This is used to make the object available to the private methods. This is a workaround for an error in the ECMAScript Language Specification which causes thisto be set incorrectly for inner functions.

Private methods cannot be called by public methods. To make private methods useful, we need to introduce a privileged method.

Privileged

A privileged method is able to access the private variables and methods, and is itself accessible to the public methods and the outside. It is possible to delete or replace a privileged method, but it is not possible to alter it, or to force it to give up its secrets.

Privileged methods are assigned with this within the constructor.

function Container(param) {

    function dec() {
        if (secret > 0) {
            secret -= 1;
            return true;
        } else {
            return false;
        }
    }

    this.member = param;
    var secret = 3;
    var that = this;

    this.service = function () {
        return dec() ? that.member : null;
    };
}

service is a privileged method. Calling myContainer.service() will return 'abc' the first three times it is called. After that, it will return null. service calls the private dec method which accesses the private secret variable. service is available to other objects and methods, but it does not allow direct access to the private members.

Closures

This pattern of public, private, and privileged members is possible because JavaScript has closures. What this means is that an inner function always has access to the vars and parameters of its outer function, even after the outer function has returned. This is an extremely powerful property of the language. It is described in How JavaScript Works.

Private and privileged members can only be made when an object is constructed. Public members can be added at any time.

Patterns

Public

function Constructor(...) {

this.membername = value;

}
Constructor.prototype.membername = value;

Private

function Constructor(...) {

var that = this;
var membername = value;

function membername(...) {...}

}

Note: The function statement

function membername(...) {...}

is shorthand for

var membername = function membername(...) {...};

Privileged

function Constructor(...) {

this.membername = function (...) {...};

}

How Javascript works 翻译系列

How Javascript works (Javascript工作原理) (一) 引擎，运行时，函数调用栈

How Javascript works (Javascript工作原理) (二) 引擎，运行时，如何在 V8 引擎中书写最优代码的 5 条小技巧

词法作用域与动态作用域

How Javascript works (Javascript工作原理) (三) 内存管理及如何处理 4 类常见的内存泄漏问题

How Javascript works (Javascript工作原理) (四) 事件循环及异步编程的出现和 5 种更好的 async/await 编程方式

How Javascript works (Javascript工作原理) (五) 深入理解 WebSockets 和带有 SSE 机制的HTTP/2 以及正确的使用姿势

How Javascript works (Javascript工作原理) (六) WebAssembly 对比 JavaScript 及其使用场景

How Javascript works (Javascript工作原理) (七) WebAssembly 对比 JavaScript 及其使用场景

How Javascript works (Javascript工作原理) (八) WebAssembly 对比 JavaScript 及其使用场景

How Javascript works (Javascript工作原理) (九) 网页消息推送通知机制 How Javascript works (Javascript工作原理) (十) 使用 MutationObserver 监测 DOM 变化

How Javascript works (Javascript工作原理) (十一) 渲染引擎及性能优化小技巧

How Javascript works (Javascript工作原理) (十二) 网络层探秘及如何提高其性能和安全性

How Javascript works (Javascript工作原理) (十三) CSS 和 JS 动画底层原理及如何优化其性能

How Javascript works (Javascript工作原理) (十五) 类和继承及 Babel 和 TypeScript 代码转换探秘

How Javascript works (Javascript工作原理) (十四) 解析，语法抽象树及最小化解析时间的 5 条小技巧

(翻译) How variables are allocated memory in Javascript? | scope chain | lexicial scope

(翻译) closures-are-not-complicated

探索JS引擎工作原理 （转)

浏览器与NodeJS环境 eventloop异同详解（转)

(翻译) closures-are-not-complicated

(翻译) Poor misunderstood 'var'

章节内容：

[
    {"number": 0, "chapter": "Read Me First!"},
    {"number": 1, "chapter": "How Names Work"},
    {"number": 2, "chapter": "How Numbers Work"},
    {"number": 3, "chapter": "How Big Integers Work"},
    {"number": 4, "chapter": "How Big Floating Point Works"},
    {"number": 5, "chapter": "How Big Big Rationals Work"},
    {"number": 6, "chapter": "How Booleans Work"},
    {"number": 7, "chapter": "How Arrays Works"},
    {"number": 8, "chapter": "How Objects Work"},
    {"number": 9, "chapter": "How Strings Work"},
    {"number": 10, "chapter": "How The Bottom Values Work"},
    {"number": 11, "chapter": "How Statements Work"},
    {"number": 12, "chapter": "How Functions Work"},
    {"number": 13, "chapter": "How Generators Work"},
    {"number": 14, "chapter": "How Exceptions Work"},
    {"number": 15, "chapter": "How Programs Work"},
    {"number": 16, "chapter": "How this Works"},
    {"number": 17, "chapter": "How Classfree Works"},
    {"number": 18, "chapter": "How Tail Calls Work"},
    {"number": 19, "chapter": "How Purity Works"},
    {"number": 20, "chapter": "How Eventual Programming Works"},
    {"number": 21, "chapter": "How Date Works"},
    {"number": 22, "chapter": "How JSON Works"},
    {"number": 23, "chapter": "How Testing Works"},
    {"number": 24, "chapter": "How Optimization Works"},
    {"number": 25, "chapter": "How Transpiling Works"},
    {"number": 26, "chapter": "How Tokenizing Works"},
    {"number": 27, "chapter": "How Parsing Works"},
    {"number": 28, "chapter": "How Code Generation Works"},
    {"number": 29, "chapter": "How Runtimes Work"},
    {"number": 30, "chapter": "How Wat! Works"},
    {"number": 31, "chapter": "How This Book Works"}
]

　　

Classical Inheritance in JavaScript

JavaScript is a class-free, object-oriented language, and as such（正因如此）, it uses prototypal inheritance instead of classical inheritance.

This can be puzzling to programmers trained in conventional object-oriented languages like C++ and Java.

JavaScript's prototypal inheritance has more expressive power than classical inheritance, as we will see presently.

But first, why do we care about inheritance at all? There are primarily two reasons.

• The first is type convenience. We want the language system to automatically cast references of similar classes.
  • Little type-safety is obtained from a type system which requires the routine explicit casting of object references. This is of critical importance in strongly-typed languages, but it is irrelevant in loosely-typed languages like JavaScript, where object references never need casting.
• The second reason is code reuse. It is very common to have a quantity of objects all implementing exactly the same methods.
  • Classes make it possible to create them all from a single set of definitions.
  • It is also common to have objects that are similar to some other objects, but differing only in the addition or modification of a small number of methods.
  • Classical inheritance is useful for this but prototypal inheritance is even more useful.

To demonstrate this, we will introduce a little sugar which will let us write in a style that resembles a conventional classical language. We will then show useful patterns which are not available in classical languages. Then finally, we will explain the sugar.

Classical Inheritance

First, we will make a Parenizor class that will have set and get methods for its value, and a toStringmethod that will wrap the value in parens.

function Parenizor(value) {
    this.setValue(value);
}

Parenizor.method('setValue', function (value) {
    this.value = value;
    return this;
});

Parenizor.method('getValue', function () {
    return this.value;
});

Parenizor.method('toString', function () {
    return '(' + this.getValue() + ')';
});

The syntax is a little unusual, but it is easy to recognize the classical pattern in it. The method method takes a method name and a function, adding them to the class as a public method.

So now we can write

myParenizor = new Parenizor(0);
myString = myParenizor.toString();

As you would expect, myString is "(0)".

Now we will make another class which will inherit from Parenizor, which is the same except that its toString method will produce "-0-" if the value is zero or empty.

function ZParenizor(value) {
    this.setValue(value);
}

ZParenizor.inherits(Parenizor);

ZParenizor.method('toString', function () {
    if (this.getValue()) {
        return this.uber('toString');//The uber method is similar to Java's super
    }
    return "-0-";
});

The inherits method is similar to Java's extends. The uber method is similar to Java's super. It lets a method call a method of the parent class. (The names have been changed to avoid reserved word restrictions.)

So now we can write

myZParenizor = new ZParenizor(0);
myString = myZParenizor.toString();

This time, myString is "-0-".

JavaScript does not have classes, but we can program as though it does.

Multiple Inheritance

By manipulating a function's prototype object, we can implement multiple inheritance, allowing us to make a class built from the methods of multiple classes.

Promiscuous multiple inheritance can be difficult to implement and can potentially suffer from method name collisions. We could implement promiscuous multiple inheritance in JavaScript, but for this example we will use a more disciplined form called Swiss Inheritance.

Suppose there is a NumberValue class that has a setValue method that checks that the value is a number in a certain range, throwing an exception if necessary. We only want its setValue and setRange methods for our ZParenizor. We certainly don't want its toString method. So, we write

ZParenizor.swiss(NumberValue, 'setValue', 'setRange');

This adds only the requested methods to our class.

Parasitic Inheritance

There is another way to write ZParenizor. Instead of inheriting from Parenizor, we write a constructor that calls the Parenizor constructor, passing off the result as its own. And instead of adding public methods, the constructor adds privileged methods.

function ZParenizor2(value) {
    var that = new Parenizor(value);
    that.toString = function () {
        if (this.getValue()) {
            return this.uber('toString');
        }
        return "-0-"
    };
    return that;
}

Classical inheritance is about the is-a relationship, and parasitic inheritance is about the was-a-but-now's-a relationship. The constructor has a larger role in the construction of the object. Notice that the uber née super method is still available to the privileged methods.

Class Augmentation

JavaScript's dynamism allows us to add or replace methods of an existing class. We can call the methodmethod at any time, and all present and future instances of the class will have that method. We can literally extend a class at any time. Inheritance works retroactively. We call this Class Augmentation to avoid confusion with Java's extends, which means something else.

Object Augmentation

In the static object-oriented languages, if you want an object which is slightly different than another object, you need to define a new class. In JavaScript, you can add methods to individual objects without the need for additional classes. This has enormous power because you can write far fewer classes and the classes you do write can be much simpler. Recall that JavaScript objects are like hashtables. You can add new values at any time. If the value is a function, then it becomes a method.

So in the example above, I didn't need a ZParenizor class at all. I could have simply modified my instance.

myParenizor = new Parenizor(0);
myParenizor.toString = function () {
    if (this.getValue()) {
        return this.uber('toString');
    }
    return "-0-";
};
myString = myParenizor.toString();

We added a toString method to our myParenizor instance without using any form of inheritance. We can evolve individual instances because the language is class-free.

Sugar

To make the examples above work, I wrote four sugar methods. First, the method method, which adds an instance method to a class.

Function.prototype.method = function (name, func) {
    this.prototype[name] = func;
    return this;
};

This adds a public method to the Function.prototype, so all functions get it by Class Augmentation. It takes a name and a function, and adds them to a function's prototype object.

It returns this. When I write a method that doesn't need to return a value, I usually have it return this. It allows for a cascade-style of programming.

Next comes the inherits method, which indicates that one class inherits from another. It should be called after both classes are defined, but before the inheriting class's methods are added.

Function.method('inherits', function (parent) {
    this.prototype = new parent();
    var d = {},
        p = this.prototype;
    this.prototype.constructor = parent;
    this.method('uber', function uber(name) {
        if (!(name in d)) {
            d[name] = 0;
        }
        var f, r, t = d[name], v = parent.prototype;
        if (t) {
            while (t) {
                v = v.constructor.prototype;
                t -= 1;
            }
            f = v[name];
        } else {
            f = p[name];
            if (f == this[name]) {
                f = v[name];
            }
        }
        d[name] += 1;
        r = f.apply(this, Array.prototype.slice.apply(arguments, [1]));
        d[name] -= 1;
        return r;
    });
    return this;
});

Again, we augment Function. We make an instance of the parent class and use it as the new prototype. We also correct the constructor field, and we add the uber method to the prototype as well.

The uber method looks for the named method in its own prototype. This is the function to invoke in the case of Parasitic Inheritance or Object Augmentation. If we are doing Classical Inheritance, then we need to find the function in the parent's prototype. The return statement uses the function's apply method to invoke the function, explicitly setting this and passing an array of parameters. The parameters (if any) are obtained from the arguments array. Unfortunately, the arguments array is not a true array, so we have to use apply again to invoke the array slice method.

Finally, the swiss method.

Function.method('swiss', function (parent) {
    for (var i = 1; i < arguments.length; i += 1) {
        var name = arguments[i];
        this.prototype[name] = parent.prototype[name];
    }
    return this;
});

The swiss method loops through the arguments. For each name, it copies a member from the parent'sprototype to the new class's prototype.

Conclusion

JavaScript can be used like a classical language, but it also has a level of expressiveness which is quite unique. We have looked at Classical Inheritance, Swiss Inheritance, Parasitic Inheritance, Class Augmentation, and Object Augmentation. This large set of code reuse patterns comes from a language which is considered smaller and simpler than Java.

Classical objects are hard. The only way to add a new member to a hard object is to create a new class. In JavaScript, objects are soft. A new member can be added to a soft object by simple assignment.

Because objects in JavaScript are so flexible, you will want to think differently about class hierarchies. Deep hierarchies are inappropriate. Shallow hierarchies are efficient and expressive.

Tips:

I have been writing JavaScript for 14 years now, and I have never once found need to use an uber function. The super idea is fairly important in the classical pattern, but it appears to be unnecessary in the prototypal and functional patterns. I now see my early attempts to support the classical model in JavaScript as a mistake.

You end up with dozens of functions in your global namespace. What we really need is a way to attach a function to an object. Since functions are objects, this is easy:

function makePerson(first, last) {
  return {
    first: first,
    last: last,
    fullName: function() {
      return this.first + ' ' + this.last;
    },
    fullNameReversed: function() {
      return this.last + ', ' + this.first;
    }
  };
}

var s = makePerson('Simon', 'Willison');
s.fullName(); // "Simon Willison"
s.fullNameReversed(); // "Willison, Simon"