极简promise雏形

function Promise(fn) {
    var value = null,
        callbacks = [];  //callbacks为数组，因为可能同时有很多个回调

    this.then = function (onFulfilled) {
        callbacks.push(onFulfilled);
    };

    function resolve(value) {
        callbacks.forEach(function (callback) {
            callback(value);
        });
    }

    fn(resolve);
}

1、定义回调函数队列callbacks数组
2、定义this.then方法，给队列添加方法 callback
3、定义resolve方法，执行回调函数callback
4、在fn里传入resolve，作为参数

转载于:https://www.cnblogs.com/jiangfengtomhuo/p/10238833.html