// 三种状态
const PENDING = "pending";
const RESOLVED = "resolved";
const REJECTED = "rejected";
// promise 接收一个函数参数，该函数会立即执行
function MyPromise(fn) {
let _this = this;
_this.currentState = PENDING;
_this.value = undefined;
// 用于保存 then 中的回调，只有当 promise
// 状态为 pending 时才会缓存，并且每个实例至多缓存一个
_this.resolvedCallbacks = [];
_this.rejectedCallbacks = [];
_this.resolve = function (value) {
if (value instanceof MyPromise) {
// 如果 value 是个 Promise，递归执行
return value.then(_this.resolve, _this.reject);
}
setTimeout(() => {
// 异步执行，必须保证执行顺序
if (_this.currentState === PENDING) {
_this.currentState = RESOLVED;
_this.value = value;
_this.resolvedCallbacks.forEach((cb) => cb());
}
});
};
_this.reject = function (reason) {
setTimeout(() => {
// 异步执行，保证执行顺序
if (_this.currentState === PENDING) {
_this.currentState = REJECTED;
_this.value = reason;
_this.rejectedCallbacks.forEach((cb) => cb());
}
});
};
// 用于解决以下问题
// new Promise(() => throw Error('error))
try {
fn(_this.resolve, _this.reject);
} catch (e) {
_this.reject(e);
}
}
MyPromise.prototype.then = function (onResolved, onRejected) {
var self = this;
// 规范 2.2.7，then 必须返回一个新的 promise
var promise2;
// 规范 2.2.onResolved 和 onRejected 都为可选参数
// 如果类型不是函数需要忽略，同时也实现了透传
// Promise.resolve(v).then().then((value) => console.log(value))
onResolved =
typeof onResolved === "function" ? onResolved : (v) => v;
onRejected = typeof onRejected === "function" ? onRejected : (r) => {throw r};
if (self.currentState === RESOLVED) {
return (promise2 = new MyPromise(function (resolve, reject) {
// 规范 2.2.4，保证 onFulfilled，onRjected 异步执行
// 所以用了 setTimeout 包裹下
setTimeout(function () {
try {
var x = onResolved(self.value);
resolutionProcedure(promise2, x, resolve, reject);
} catch (reason) {
reject(reason);
}
});
}));
}
if (self.currentState === REJECTED) {
return (promise2 = new MyPromise(function (resolve, reject) {
setTimeout(function () {
// 异步执行onRejected
try {
var x = onRejected(self.value);
resolutionProcedure(promise2, x, resolve, reject);
} catch (reason) {
reject(reason);
}
});
}));
}
if (self.currentState === PENDING) {
return (promise2 = new MyPromise(function (resolve, reject) {
self.resolvedCallbacks.push(function () {
// 考虑到可能会有报错，所以使用 try/catch 包裹
try {
var x = onResolved(self.value);
resolutionProcedure(promise2, x, resolve, reject);
} catch (r) {
reject(r);
}
});
self.rejectedCallbacks.push(function () {
try {
var x = onRejected(self.value);
resolutionProcedure(promise2, x, resolve, reject);
} catch (r) {
reject(r);
}
});
}));
}
};
// 规范 2.3
function resolutionProcedure(promise2, x, resolve, reject) {
// 规范 2.3.1，x 不能和 promise2 相同，避免循环引用
if (promise2 === x) {
return reject(new TypeError("Error"));
}
// 规范 2.3.2
// 如果 x 为 Promise，状态为 pending 需要继续等待否则执行
if (x instanceof MyPromise) {
if (x.currentState === PENDING) {
x.then(function (value) {
// 再次调用该函数是为了确认 x resolve 的
// 参数是什么类型，如果是基本类型就再次 resolve
// 把值传给下个 then
resolutionProcedure(promise2, value, resolve, reject);
}, reject);
} else {
x.then(resolve, reject);
}
return;
}
// 规范 2.3.3.3.3
// reject 或者 resolve 其中一个执行过得话，忽略其他的
let called = false;
// 规范 2.3.3，判断 x 是否为对象或者函数
if (x !== null && (typeof x === "object" || typeof x === "function")) {
// 规范 2.3.3.2，如果不能取出 then，就 reject
try {
// 规范 2.3.3.1
let then = x.then;
// 如果 then 是函数，调用 x.then
if (typeof then === "function") {
// 规范 2.3.3.3
then.call(
x,
(y) => {
if (called) return;
called = true;
// 规范 2.3.3.3.1
resolutionProcedure(promise2, y, resolve, reject);
},
(e) => {
if (called) return;
called = true;
reject(e);
}
);
} else {
// 规范 2.3.3.4
resolve(x);
}
} catch (e) {
if (called) return;
called = true;
reject(e);
}
} else {
// 规范 2.3.4，x 为基本类型
resolve(x);
}
}async/await基于promise的语法糖：
async函数返回一个promise对象
const async = (func) => {
const p = new Promise((resolve, reject) => {
try {
const value = func()
if (
((typeof value === 'object' && value !== null) || typeof value === 'function') &&
typeof value.then === 'function') {
Promise.resolve(value).then(resolve, reject)
} else {
resolve(value)
}
} catch (error) {
reject(error)
}
})
return p
}
const await = (arg) => (onResolved, onRejected) => {
const innerPromise = onRejected ? Promise.resolve(arg).catch(onRejected).then(onResolved, onRejected)
: Promise.resolve(arg).then(onResolved, onRejected)
return innerPromise
}