手撕Promise

手撕Promise

十月 11, 2020
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// 手写Promise

class MyPromise {
constructor(fn) {
if (typeof fn !== "function") {
throw new Error('MyPromise Expect A Function')
}

this.status = 'Pending';
this.onResolvedCallback = [];
this.onRejectedCallback = [];
this.val = undefined;

const resolve = (val) => {
setTimeout(() => {
console.log(this.status, this.onResolvedCallback)
if(this.status === 'Pending') {
this.status = 'Resolved'
this.val = val;
this.onResolvedCallback.forEach(item => item(val))
}
}, 0)
}

const reject = (val) => {
setTimeout(() => {
if(this.status === 'Pending') {
this.status = 'Rejected'
this.val = val;
this.onRejectedCallback.forEach(item => item(val))
}
}, 0)
}

try {
fn(resolve, reject)
} catch (e) {
reject(e);
}


return this.status;
}

then(onFulfilled, onRejected) {
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : o => o;
onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };

return new MyPromise((resolve, reject) => {
this.onResolvedCallback.push((val) => {
try {
const res = onFulfilled(val)
if(res instanceof MyPromise) {
res.then(resolve, reject)
} else {
resolve(res);
}
} catch (e) {
reject(e);
}
})

this.onRejectedCallback.push((val) => {
try {
const res = onRejected(val)
if(res instanceof MyPromise) {
res.then(resolve, reject)
} else {
reject(res);
}
} catch (e) {
reject(e);
}
})
})
}

catch(onRejected) {
return this.then(null, onRejected);
}
}

const p = new MyPromise((res, rej) => {res(1)})

p.then(3)
.then(cal => {
console.log(cal)
})
.catch(err=> {
console.log(err)
})