class MyPromise {
constructor(fn) {
try {
fn((res) => this.resolve(res), (err) => this.reject(err))
} catch (error) {
this.reject(error)
}
}
"[[PromiseState]]" = 'pending';
resolve(res) {
this["[[PromiseState]]"] = 'fulfilled'
this.run(() => this.each(res, this._thenList))
};
reject(err) {
this["[[PromiseState]]"] = 'rejected'
this.run(() => this.each(err, this._catchList))
};
run(fn) {
if (typeof MutationObserver !== 'undefined') {
let dom = document.createElement('div')
const ob = new MutationObserver(() => {
ob.disconnect();
fn();
dom = undefined;
})
ob.observe(dom, { attributes: true })
dom.setAttribute('a', 2)
} else if (typeof process !== 'undefined') {
process.nextTick(fn);
} else {
setTimeout(fn)
}
}
each(_res, list) {
let res = _res;
let fn = list.shift();
const walk = () => {
if (!fn) return;
this.checkRes(fn(res), newRes => {
res = newRes;
fn = list.shift();
walk();
})
}
walk()
}
checkRes(res, callback) {
if (res?.then) {
res.then(_res => {
callback(_res)
})
} else {
callback(res);
}
}
_thenList = [];
then(fn) {
this._thenList.push(fn)
return this;
}
_catchList = [];
catch(fn) {
this._catchList.push(fn)
return this;
}
}
const myPromise = new MyPromise((resolve, reject) => {
resolve(1)
});
setTimeout(() => console.log('setTimeout'))
myPromise.then(res => res + 44).then(res => {
console.log(res);
}).catch(err => {
console.log('err', err);
})
