Expressions versus statements in JavaScript

Statements and expressions

An expression produces a value and can be written wherever a value is expected.

Expressions that look like statements

• Expressions that look like statements

  JavaScript has stand-alone blocks? It might surprise you that JavaScript has blocks that can exist on their own (as opposed to being part of a loop or an if statement). The following code illustrates one use case for such blocks: You can give them a label and break from them.

      function test(printTwo) {
        printing: {
            console.log("One");
            if (!printTwo) break printing;
            console.log("Two");
        }
        console.log("Three");
    }
  

• Function expression versus function declaration
  
  function expressions

function() {}
function fn() {}

Using object literals and function expressions as statements

• eval parses its argument in statement context. If you want eval to return an object, you have to put parentheses around an object literal.

    > eval("{ foo: 123 }")
    123
    > eval("({ foo: 123 })")
    { foo: 123 }
  

• Immediately invoked function expressions (IIFEs)

    > (function () { return "abc" }())
    'abc'
  

  If you omit the parentheses, you get a syntax error (function declarations can’t be anonymous):

    > function () { return "abc" }()
    SyntaxError: function statement requires a name
  

  If you add a name, you also get a syntax error (function declarations can’t be immediately invoked):

    > function foo() { return "abc" }()
    SyntaxError: syntax error
  

  Another way of guaranteeing that an expression is parsed in expression context is a unary operator such as + or !. But, in contrast to parentheses, these operators change the result of the expression. Which is OK, if you don’t need it:

    > +function () { console.log("hello") }()
    hello
    NaN
  

  NaN is the result of applying + to undefined, the result of calling the function. Brandon Benvie mentions another unary operator that you can use: void [2]:

    > void function () { console.log("hello") }()
    hello
    undefined
  

• Concatenating IIFEs

  When you concatenate IIFEs, you must be careful not to forget semicolons:

    (function () {}())
    (function () {}())
    // TypeError: undefined is not a function
  

  This code produces an error, because JavaScript thinks that the second line is an attempt to call the result of the first line as a function. The fix is to add a semicolon:

    (function () {}());
    (function () {}())
    // OK
  

  With operators that are only unary (plus is both unary and binary), you can omit the semicolon, because automatic semicolon insertion kicks in.

    void function () {}()
    void function () {}()
    // OK
  

  JavaScript inserts a semicolon after the first line, because void is not a valid way of continuing this statement [3].

原文：Expressions versus statements in JavaScript