js promise实现原理

Posted 2022-02-04 vieber

简单实现版本

class Promise 
    callbacks = [];
    state = 'pending';
    value = null;
    constructor(fn) 
        fn(this._resolve.bind(this))
    
    then(onFulfilled) 
        if (state === 'pending') 
            this.callbacks.push(onFulfilled);
           
        else 
            onFulfilled(this.value);
        
        
        return this;
    
    _resolve(value) 
        this.state = 'fulfilled';
        this.value = value;
        this.callbacks.forEach(fn => fn(value));