Promises/A+ 规范文档
代码实现
enum States {
PENDING = "pending",
FULFILLED = "fulfilled",
REJECTED = "rejected",
}
interface Resolve<T> {
(value: T | PromiseLike<T>): void;
}
interface Reject {
(reason?: any): void;
}
interface Executor<T> {
(resolve: Resolve<T>, reject: Reject): void;
}
// PromisesA+ 1.1
interface PromiseLike<T> {
then<TResult1 = T, TResult2 = never>(
// PromisesA+ 2.2.1
onFulfilled?: ((value: T | PromiseLike<T>) => TResult1 | PromiseLike<TResult1>) | undefined | null,
onRejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null
): PromiseLike<TResult1 | TResult2>;
}
export class MyPromise<T> {
private state: States = States.PENDING;
private onFulfilledCallbacks = [] as (() => void)[]; // 成功时的回调函数
private onRejectedCallbacks = [] as (() => void)[]; // 失败时的回调函数
private value!: T | PromiseLike<T>;
private reason: any;
constructor(executor: Executor<T>) {
try {
executor(this.resolve, this.reject);
} catch (e) {
// PromisesA + 1.4
this.reject(e);
}
}
// PromisesA+ 1.3 | PromisesA+ 2.1
private resolve: Resolve<T> = (value: T | PromiseLike<T>) => {
try {
// PromisesA+ 2.2.4 异步调用，用setTimeout模拟创建微任务
setTimeout(() => {
if (this.state === States.PENDING) {
this.state = States.FULFILLED;
this.value = value;
// PromisesA+ 2.2.6.1
this.onFulfilledCallbacks.forEach((fn) => fn());
this.onFulfilledCallbacks = [];
}
});
} catch (e) {
this.reject(e);
}
};
// PromisesA+ 1.5
private reject: Reject = (reason: any) => {
try {
// PromisesA+ 2.2.4 异步调用，用setTimeout模拟创建微任务
setTimeout(() => {
if (this.state === States.PENDING) {
this.state = States.REJECTED;
this.reason = reason;
// PromisesA+ 2.2.6.2
this.onRejectedCallbacks.forEach((fn) => fn());
this.onRejectedCallbacks = [];
}
});
} catch (e) {
this.reject(e);
}
};
// PromisesA+ 1.2 | PromisesA+ 2.2
then<TResult1 = T, TResult2 = never>(
// PromisesA+ 2.2.1
onFulfilled?: ((value: T | PromiseLike<T>) => TResult1 | PromiseLike<TResult1>) | undefined | null,
onRejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null
): MyPromise<TResult1 | TResult2> {
// PromisesA+ 2.2.5 | PromisesA+ 2.2.7.3 | PromisesA+ 2.2.7.4
onFulfilled = typeof onFulfilled === "function" ? onFulfilled : (val: T | PromiseLike<T>) => val as any;
onRejected =
typeof onRejected === "function"
? onRejected
: (r: any) => {
throw r;
};
// PromisesA+ 2.2.7
const promise2 = new MyPromise<TResult1 | TResult2>((resolve: Resolve<TResult1 | TResult2>, reject: Reject) => {
if (this.state === States.FULFILLED) {
// PromisesA+ 2.2.2
// PromisesA+ 2.2.4 异步调用，用setTimeout模拟创建微任务
setTimeout(() => {
try {
// PromisesA+ 2.2.7.1
let x = onFulfilled!(this.value);
this.resolvePromise(promise2, x, resolve, reject);
} catch (e) {
// PromisesA+ 2.2.7.2
reject(e);
}
});
} else if (this.state === States.REJECTED) {
// PromisesA+ 2.2.3
// PromisesA+ 2.2.4 异步调用，用setTimeout模拟创建微任务
setTimeout(() => {
try {
// PromisesA+ 2.2.7.1
let x = onRejected!(this.reason);
this.resolvePromise(promise2, x, resolve, reject);
} catch (e) {
// PromisesA+ 2.2.7.2
reject(e);
}
});
} else if (this.state === States.PENDING) {
// PromisesA+ 2.2.6
// 调用回调函数时是异步的，因此这里不再需要加setTimeout
this.onFulfilledCallbacks.push(() => {
try {
let x = onFulfilled!(this.value);
// PromisesA+ 2.2.7.1
this.resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
});
this.onRejectedCallbacks.push(() => {
try {
let x = onRejected!(this.reason);
// PromisesA+ 2.2.7.1
this.resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
});
}
});
return promise2;
}
resolvePromise<T>(promise: MyPromise<T>, x: T | PromiseLike<T>, resolve: Resolve<T>, reject: Reject) {
// PromisesA+ 2.3.1
if (promise === x) {
const e = new TypeError("TypeError: Circular reference");
reject(e);
}
let called = false; // 防止resolve和reject多次调用
// PromisesA+ 2.3.3
if (x && (typeof x === "object" || typeof x === "function")) {
try {
// PromisesA+ 2.3.3.1 | PromisesA+ 2.3.2
const then = (x as PromiseLike<T>).then;
if (typeof then === "function") {
then.call(
x,
// PromisesA+ 2.3.3.3.1
(y: T | PromiseLike<T>) => {
// PromisesA+ 2.3.3.3.3
if (called) return;
called = true;
// 直到解析的对象不再是 thenable，取出其中的值
this.resolvePromise(promise, y, resolve, reject);
},
// PromisesA+ 2.3.3.3.2
(r: any) => {
// PromisesA+ 2.3.3.3.3
if (called) return;
called = true;
reject(r);
}
);
} else {
// PromisesA+ 2.3.3.4
resolve(x);
}
} catch (e) {
// PromisesA+ 2.3.3.2 | PromisesA+ 2.3.3.3.3 | PromisesA+ 2.3.3.3.4
if (called) return;
called = true;
reject(e);
}
} else {
// PromisesA+ 2.3.4
resolve(x);
}
}
}
测试流程
测试工具文档：Promises/A+ Compliance Test Suite
为了使用官方工具进行测试，需要在程序的最后添加如下代码：
// @ts-ignore
MyPromise.defer = MyPromise.deferred = function () {
let deferred: any = {};
deferred.promise = new MyPromise((resolve, reject) => {
deferred.resolve = resolve;
deferred.reject = reject;
});
return deferred;
};
// @ts-ignore
export = MyPromise;
将ts代码编译为js
tsc promise.ts
运行测试
npx promises-aplus-tests promise.js
测试结果: