https://juejin.im/post/5b2f02cd5188252b937548ab 详解
https://juejin.im/post/5cd8d76d5188256805093a9f
//Promise函数
function Promise(executor) {
let self = this; //保留this。防止后面方法出现this只想不明的问题
self.status = 'pending'; //promise的默认状态是pending
self.value= undefined; //保存成功回调传递的值
self.error = undefined; //保存失败回调传递的值
self.onSuccessCallbacks = []; //存放成功的回调
self.onErrorCallbacks = []; //存放失败的回调
function resolve(value) {
if (self.status === 'pending') {
self.status = 'resolved'; //成功函数将其状态修改为resolved
self.value= value; //将成功的值保存起来
self.onSuccessCallbacks.forEach(fn=> {
fn();
});
}
}
function reject(error) {
if (self.status === 'pending') {
self.status = 'rejected'; //失败函数将其函数修改为rejected
self.error = error; //将失败的值保存起来
self.onErrorCallbacks.forEach(fn=> {
fn();
})
}
}
try {
executor(resolve, reject);
} catch (err) {
reject(err);
}
}
//then函数
Promise.prototype.then = function (onResolved, onRejected) {
let self = this;
let promiseAgain = new Promise((resolve, reject) => {
if (self.status === 'pending') {
self.onSuccessCallbacks.push(() => {
let x = onResolved(self.value); //将resolve函数保留的成功值传递作为参数
resolvePromise(promiseAgain, x, resolve, reject);
})
self.onErrorCallbacks.push(() => {
let x = onRejected(self.error); //将reject函数保留的失败值传递作为参数
resolvePromise(promiseAgain, x, resolve, reject);
})
}
if (self.status === 'resolved') {
let x = onResolved(self.value); //将resolve函数保留的成功值传递作为参数
resolvePromise(promiseAgain, x, resolve, reject);
}
if (self.status === 'rejected') {
let x = onRejected(self.error); //将reject函数保留的失败值传递作为参数
resolvePromise(promiseAgain, x, resolve, reject);
}
})
return promiseAgain;
}
//resolvePromise函数
function resolvePromise(promiseAgain, x, resolve, reject) {
if (promiseAgain === x) {
return reject(new TypeError("循环调用"));
}
if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
try {
let then = x.then;
if (typeof then === 'function') {
then.call(x, (y) => {
resolvePromise(promiseAgain, y, resolve, reject);
}, (e) => {
reject(e);
})
} else {
resolve(x);
}
} catch (error) {
reject(error);
}
} else {
resolve(x);
}
}
module.exports = Promise;
