// 例2

​

function getUserId() {

    return new Promise(function (resolve) {

        // 模拟异步操作

        window.setTimeout(function () {

            resolve(9876);

        }, 1000);

    });

}

​

getUserId().then(function (id) {

    console.log('do sth with', id);

}).then(function (id) {

    console.log('do sth else with', id);

});

​

​

function Promise(fn) {

    var value = null,

        deferreds = [];

    this.then = function (onFulfilled) {

        deferreds.push(onFulfilled);

        return this;

    };

    function resolve(value) {

        deferreds.forEach(function (deferred) {

            deferred(value);

        });

    }

​

    fn(resolve);

}