webpack基础小结。
参考 Promise A+ 简单实现 promise,用的setTimeout 模拟的异步,与实际浏览器Promise 有出入(具体可以看Event loop 相关),本文只做思路理解参考。
Promise 的简单实现,主要理解规范,也可以再理解一下几个点方变记忆:
- Promise的三种状态(pending,fulfilled,rejected), pending状态转移后不可变
- resolve , resolve a promise with a promise
- reject
- Promise.prototype.then 返回一个新的promise 对象(链式操作)
- thenable对象
1.简单实现
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected' function needResolve() {
throw new TypeError('need pass function to Promise')
} function needuseNew() {
throw new TypeError(
'Promise is a constructor and should be called with the `new` keyword'
)
} function Promise(task) {
var self = this
this.state = PENDING
this.value
this.fulArr = []
this.rejArr = [] function resolve(value) {
if (value != null && value.then && typeof value.then == 'function') {
return value.then(resolve, reject)
}
setTimeout(() => {
if (self.state == PENDING) {
self.state = FULFILLED
self.value = value
self.fulArr.forEach(item => {
item(self.value)
})
}
})
} function reject(reason) {
setTimeout(() => {
if (self.state == PENDING) {
self.state = REJECTED
self.value = reason
self.fulArr.forEach(item => {
item(self.value)
})
}
})
}
typeof task !== 'function' && needResolve()
!(this instanceof Promise) && needResolve()
try {
task(
function(value) {
resolve(value)
},
function(reason) {
reject(reason)
}
)
} catch (e) {
reject(e)
}
} function resolvePromise(promise, x, resolve, reject) {
if (promise === x) {
return reject(new TypeError('do not resolve promise with itself'))
}
var then, called
// resolve promise with promise
if (x instanceof Promise) {
if (x.status == PENDING) {
x.then(function(y) {
resolvePromise(promise, y, resolve, reject)
}, reject)
} else {
x.then(resolve, reject)
}
// thenable 其它的then
} else if (x !== null && (typeof x == 'function' || typeof x == 'object')) {
try {
// resolve promise with promise
then = x.then
if (typeof then === 'function') {
then.call(
x,
function(y) {
if (called) {
return
}
called = true
resolvePromise(promise, y, resolve, reject)
},
function(r) {
if (called) {
return
}
called = true
reject(r)
}
)
} else {
resolve(x)
}
} catch (e) {
if (called) {
return
}
called = true
reject(e)
}
} else {
resolve(x)
}
}
Promise.prototype.then = function(onFulfilled, onRejected) {
var self = this
onFulfilled =
typeof onFulfilled === 'function'
? onFulfilled
: function(value) {
return value
}
onRejected =
typeof onRejected === 'function'
? onRejected
: function(reason) {
throw reason
}
var promise2
if (self.state == PENDING) {
promise2 = new Promise(function(resolve, reject) {
self.fulArr.push(value => {
try {
let x = onFulfilled(value)
resolvePromise(promise2, x, resolve, reject)
} catch (e) {
reject(e)
}
})
self.reject.push(reason => {
try {
let x = onRejected(reason)
resolvePromise(promise2, x, resolve, reject)
} catch (e) {
reject(e)
}
})
})
}
if (self.state == FULFILLED) {
promise2 = new Promise(function(resolve, reject) {
setTimeout(() => {
try {
let x = onFulfilled(self.value)
resolvePromise(promise2, x, resolve, reject)
} catch (e) {
reject(e)
}
})
})
}
if (self.state == REJECTED) {
promise2 = new Promise(function(resolve, reject) {
setTimeout(() => {
try {
let x = onRejected(self.value)
resolvePromise(promise2, x, resolve, reject)
} catch (e) {
reject(e)
}
})
})
}
// then 必须返回一个新的promise
return promise2
} Promise.prototype.catch = function(onRejected) {
return this.then(null, onRejected)
} Promise.resolve = function(value) {
return new Promise((resolve, reject) => {
resolve(value)
})
}
Promise.reject = function(value) {
return new Promise((resolve, reject) => {
reject(value)
})
} function gen(time, resolve) {
let result = []
let i = 0
return function(index, value) {
if (i < time) {
i++
result[index] = value
} else {
resolve(result)
}
}
} Promise.race = function(arr) {
return new Promise((resolve, reject) => {
for (let i = 0; i < arr.length; i++) {
Promise.resolve(arr[i]).then(resolve, reject)
}
})
}
Promise.all = function(arr) {
return new Promise((resolve, reject) => {
let done = gen(arr.length, resolve)
for (let i = 0; i < arr.length; i++) {
Promise.resolve(arr[i]).then(function(value) {
done(value, i)
}, reject)
}
})
}
module.exports = Promise
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