Promise标准了解一下
传送门️ Promises/A+规范
传送门️ 阮一峰前辈ECMAScript 6入门——Promise对象
Promise重点难点
Promise的状态不受外界影响，只由Promise内的异步操作结果决定。
Promise的状态一旦改变就不会再变。
pending fulfilled
pending rejected
Promise的构造函数是同步的，then()方法中的函数是异步的。
then()方法或者catch()方法的参数期望是函数，传入非函数则会发生值穿透
Promise属于microtask，同一次事件循环中，microtask永远在macrotask之前执行。
手写promise
实现基本状态（pending, fulfilled, rejected）
实现then方法
返回一个promise，实现链式调用
实现状态判断
实现resolve函数
接受一个Promise作为参数时的情况，实现链式Promise
执行回调函数时使用setTimeout，保证在注册then方法后触发
实现Promise.all,Promise.race方法
(function (window) { var Promise = function (fn) { var value = null; var callbacks = []; var state = 'pending'; // pending, fulfilled, rejected
var promise = this; // 注册then事件，供resolve后调用
promise.then = function (onFulfilled, onRejected) { // 返回promise实现链式promise调用
return new Promise(function (resolve, reject) {
handle({ onFulfilled: onFulfilled || null, onRejected: onRejected || null, resolve: resolve, reject: reject
})
})
}
promise.catch = function(onRejected) { return promise.then(undefined, onRejected)
}
function handle (callback) { // 状态变化前，事件推进队列里；状态一旦变化后不再变动，直接执行结果
if (state === 'pending') {
callbacks.push(callback);
} else { var cb = state === 'fulfilled' ? callback.onFulfilled : callback.onRejected // then方法没有传递任何参数的情况下，返回结果值
if (!cb) {
cb = state === 'fulfilled' ? callback.resolve : callback.reject;
cb(value)
} else { try { var ret = cb(value);
callback.resolve(ret);
} catch (e) {
callback.reject(e)
}
}
}
} function resolve(newValue) { // 状态一旦改变便不再改变
if (state !== 'pending') return
// 假如resolve了一个promise的话（链式promise）
if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) { var then = newValue.then; if (typeof then === 'function') { // 调用第二个promise中的then，递归直到不是一个promise值为止
then.call(newValue, resolve, reject); return
}
}
value = newValue;
state = 'fulfilled';
execute()
} function reject(reason) { // 状态一旦改变便不再改变
if (state !== 'pending') return
state = 'rejected'
value = reason
execute()
} function execute() { // 使用setTimeOut保证resolve一定在then事件注册后执行
setTimeout(() => {
callbacks.forEach(function (callback) {
handle(callback);
})
}, 0);
}
fn(resolve, reject);
} Promise.all = function (promises) { if (!Array.isArray(promises)) { throw new TypeError('请传入promise数组')
} return new Promise(function (resolve, reject) { var result = []; var count = promises.length function reslover (index) { return function(value) {
resloveAll(index, value)
}
}
function rejecter (reason) {
reject(reason)
}
function resloveAll (index, value) {
result[index] = value // 等待全部promise执行完才执行resolve一个数组
if (--count === 0) {
resolve(result)
}
}
promises.forEach(function (promise, index) {
promise.then(reslover(index), rejecter)
})
})
} Promise.race = function (promises) { if (!Array.isArray(promises)) { throw new TypeError('请传入promise数组')
} return new Promise(function (resolve, reject) { function reslover (value) {
resolve(value)
}
function rejecter (reason) {
reject(reason)
}
promises.forEach(function (promise, index) {
promise.then(reslover, rejecter)
})
})
} window.Promise = Promise})(window)/******************************************实例 */function test(i) { return new Promise(function (resolve) {
setTimeout(() => {
resolve(i);
}, 1000);
})
}function test2(i) { return new Promise(function (resolve, reject) {
setTimeout(() => { if (i % 2) {
resolve(i);
} else {
reject(i);
}
}, 2000);
})
}// 链式Promisetest(1).then(test2).then(function (something) { console.log('case1: success!' + something);
}).catch(function (something) { console.log('case1: failed!' + something);
})// Promise.allPromise.all([test(2), test2(4)]).then(function (something) { console.log('case2: success!' + something);
}).catch(function (something) { console.log('case2: failed!' + something)
})// Promise.racePromise.race([test(3), test2(5)]).then(function (something) { console.log('case3: success!' + something);
}).catch(function (something) { console.log('case3: failed!' + something)
})// 多次改变状态，只有第一次生效function test3 () { return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('success1')
reject('error')
resolve('success2')
},0)
})
}
test3().then((res) => { console.log('then: ', res)
})// 一旦改变状态后，多次调用then()方法也只会马上得到Promise返回的值，Promise构造函数不会多次执行function test4 () { return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('success')
}, 1000)
})
}
const start = Date.now()
test4().then((res) => { console.log(res, Date.now() - start)
})
test4().then((res) => { console.log(res, Date.now() - start)
})
小礼物走一走，来简书
作者：Eason_Wong
链接：https://www.jianshu.com/p/b7eba0d43af9
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