// 例3

​

function getUserId() {

    return new Promise(function (resolve) {

        resolve(123);

    });

}

​

var promise = getUserId();

window.setTimeout(function () {

    promise.then(function (id) {

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

    });

}, 1000);

​

​

function Promise(fn) {

    var state = 'pending',

        value = null,

        deferreds = [];

    this.then = function (onFulfilled) {

        if (state === 'pending') {

            deferreds.push(onFulfilled);

            return this;

        }

        onFulfilled(value);

        return this;

    };

    function resolve(newValue) {

        value = newValue;

        state = 'fulfilled';

        setTimeout(function () {

            deferreds.forEach(function (deferred) {

                deferred(value);

            });

        }, 0);

    }

​

    fn(resolve);

}

​