//promise里面只有三个状态，且三个状态的转换形式有两种
//由pending转换为fulfilled，由pending转换为rejected
//Promise的构造函数参数是一个函数，函数的参数分别为resolve和reject，两者也均为一个函数
//then中是实际要执行的函数，将传递进来的值传给resolve和reject对应的参数
const PENDING = 'PENDING'
const FULFILLED = 'FULFILLED'
const REJECTED = 'REJECTED'
class YPromise {
constructor(cb) {
this.state = PENDING;
this.value = null;
//承诺完成的回调列表
this.fulfilledCbs = [];
//承诺被拒绝的回调列表
this.rejectedCbs = [];
let resolve = data => {
setTimeout(() => {
if (this.state !== PENDING) return;
//更改状态
this.state = FULFILLED;
this.value = data;
this.fulfilledCbs.forEach(c => {
this.value = c(this.value);
})
}, 0);
}
let reject = reason => {
setTimeout(() => {
if (this.state !== PENDING) return;
this.state = REJECTED;
this.reason = reason;
this.rejectedCbs.forEach(c => {
this.reason = c(this.reason);
})
}, 0);
}
cb(resolve, reject);
};
then(onFulfilled, onRejected) {
if (typeof onFulfilled === 'function') {
this.fulfilledCbs.push(onFulfilled);
}
if (typeof onRejected === 'function') {
this.rejectedCbs.push(onRejected);
}
return this;//返回整个构造函数可以继续调用then方法
}
}
let promise = new YPromise((resolve, reject) => {
if (4 > 1) {
resolve('hi');
} else {
reject(4大于1')
}
})
fulfilledCbs = [data => data + 'world']
promise.then(data => {
return data + ' world';
}).then(data => {
return data + '!';
}).then(data => {
console.log(data);
})
