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//极简的实现+链式调用+延迟机制+状态
class Promise {
    callbacks = [];
    state = 'pending';//增加状态
    value = null;//保存结果
    constructor(fn) {
        fn(this._resolve.bind(this));
    }
    then(onFulfilled) {
        if (this.state === 'pending') {//在resolve之前，跟之前逻辑一样，添加到callbacks中
            this.callbacks.push(onFulfilled);
        } else {//在resolve之后，直接执行回调，返回结果了
            onFulfilled(this.value);
        }
        return this;
    }
    _resolve(value) {
        this.state = 'fulfilled';//改变状态
        this.value = value;//保存结果
        this.callbacks.forEach(fn => fn(value));
    }
}

源码

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//完整的实现
class Promise {
    callbacks = [];
    state = 'pending';//增加状态
    value = null;//保存结果
    constructor(fn) {
        fn(this._resolve.bind(this));
    }
    then(onFulfilled) {
        return new Promise(resolve => {
            this._handle({
                onFulfilled: onFulfilled || null,
                resolve: resolve
            });
        });
    }
    _handle(callback) {
        if (this.state === 'pending') {
            this.callbacks.push(callback);
            return;
        }
        //如果then中没有传递任何东西
        if (!callback.onFulfilled) {
            callback.resolve(this.value);
            return;
        }
        var ret = callback.onFulfilled(this.value);
        callback.resolve(ret);
    }
    _resolve(value) {
        this.state = 'fulfilled';//改变状态
        this.value = value;//保存结果
        this.callbacks.forEach(callback => this._handle(callback));
    }
}