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| const stateMap = {
PENDING: 'pending',
FULFILLED: 'fulfilled',
REJECTED: 'rejected',
}
class MyPromise {
constructor(executor) {
this.id = id;
this.state = stateMap.PENDING;
this.value = undefined;
this.reason = undefined;
this.fulfillCallbackList = [];
this.rejectCallbackList = [];
const resolve = (value) => {
if (this.state !== stateMap.PENDING) return;
this.state = stateMap.FULFILLED;
this.value = value;
this.fulfillCallbackList.forEach((callback) => callback(this.value));
}
const reject = (reason) => {
if (this.state !== stateMap.PENDING) return;
this.state = stateMap.REJECTED;
this.reason = reason;
this.rejectCallbackList.forEach((callback) => callback(this.reason));
}
try {
executor(resolve, reject);
} catch (error) {
reject(error);
}
}
then(onFulFill, onReject) {
const thenPromise = new MyPromise((resolve, reject) => {
if (this.state === stateMap.FULFILLED) {
try {
const value = onFulFill(this.value);
resolve(value);
} catch (error) {
reject(error);
}
} else if (this.state === stateMap.REJECTED) {
try {
const reason = onReject(this.reason);
reject(reason);
} catch (error) {
reject(error);
}
} else {
this.fulfillCallbackList.push(() => {
try {
const value = onFulFill(this.value);
resolve(value);
} catch (error) {
reject(error);
}
});
this.rejectCallbackList.push(() => {
try {
const reason = onReject(this.reason);
resolve(reason);
} catch (error) {
reject(error);
}
})
}
});
return thenPromise;
}
catch(rejectCallback) {
return this.then(null, rejectCallback);
}
}
function test(PromiseType = Promise) {
const promise = new PromiseType((resolve, reject) => {
setTimeout(() => {
const num = Math.random() * 10;
if (num < 5) {
resolve(num);
} else {
reject('num is not less than 5');
}
}, 3000);
});
promise.then((value) => {
console.log('fulfill-1', value);
value = `then1-${value}`;
},(reason) => {
console.log('reject-1', reason);
reason = `then1-${reason}`;
return reason;
}).then((value)=> {
console.log('fulfill-2', value);
value = `then2-${value}`;
}, (reason) => {
console.log('reject-2', reason);
reason = `then2-${reason}`;
}).then((value) => {
console.log('promise', promise, value);
});
}
test(Promise);
test(MyPromise);
|
