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