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promise.js
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promise.js
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// 手写promise
class MyPromise {
constructor(executor) {
this.initValue();
this.initBind();
try {
executor(this.resolve, this.reject);
} catch (error) {
this.reject(error);
}
}
initValue() {
this.promiseState = 'pending'; // 状态
this.promiseResult = null; // 结果
this.onFulfilledCallbacks = []; // 成功回调
this.onRejectedCallbacks = []; // 失败回调
}
initBind() {
this.resolve = this.resolve.bind(this);
this.reject = this.reject.bind(this);
}
resolve(value) {
if (this.promiseState !== 'pending') return;
this.promiseState = 'fulfilled';
this.promiseResult = value;
while (this.onFulfilledCallbacks.length) {
this.onFulfilledCallbacks.shift()(this.promiseResult);
}
}
reject(value) {
if (this.promiseState !== 'pending') return;
this.promiseState = 'rejected';
this.promiseResult = value;
while (this.onRejectedCallbacks.length) {
this.onRejectedCallbacks.shift()(this.promiseResult);
}
}
then(onFulfilled, onRejected) {
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : (res) => res;
onRejected =
typeof onRejected === 'function'
? onRejected
: (reason) => {
throw reason;
};
const returnPromise = new MyPromise((resolve, reject) => {
setTimeout(() => {
const resolvePromise = (cb) => {
try {
const x = cb(this.promiseResult);
if (x === returnPromise) {
reject('不能返回自己');
} else if (x instanceof MyPromise) {
x.then(resolve, reject);
} else {
resolve(x);
}
} catch (error) {
reject(error);
}
};
if (this.promiseState === 'fulfilled') {
resolvePromise(onFulfilled);
} else if (this.promiseState === 'rejected') {
resolvePromise(onRejected);
} else if (this.promiseState === 'pending') {
this.onFulfilledCallbacks.push(resolvePromise.bind(this, onFulfilled));
this.onRejectedCallbacks.push(resolvePromise.bind(this, onRejected));
}
}, 0);
});
return returnPromise;
}
}
const pro = new MyPromise((resolve) => {
resolve(123);
});
pro
.then((res) => {
console.log(res);
return new MyPromise((resolve) => {
resolve(999);
});
})
.then((res) => {
console.log(res);
});
const pro2 = new Promise((resolve) => {
resolve(123);
});
pro2
.then((res) => {
console.log(res);
return new Promise((resolve) => {
resolve(999);
});
})
.then((res) => {
console.log(res);
});