// 例4

​

function getUserId() {

    return new Promise(function (resolve) {

        window.setTimeout(function () {

            resolve(9876);

        });

    });

}

​

function getUserMobileById(id) {

    return new Promise(function (resolve) {

      console.log('start to get user mobile by id:', id);

        window.setTimeout(function () {

            resolve(13810001000);

        });

    });

}

​

getUserId()

    .then(getUserMobileById)

    .then(function (mobile) {

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

    });

​

​

function Promise(fn) {

    var state = 'pending',

        value = null,

        deferreds = [];

    this.then = function (onFulfilled) {

        return new Promise(function (resolve) {

            handle({

                onFulfilled: onFulfilled || null,

                resolve: resolve

            });

        });

    };

​

    function handle(deferred) {

        if (state === 'pending') {

            deferreds.push(deferred);

            return;

        }

        var ret = deferred.onFulfilled(value);

        deferred.resolve(ret);

    }

    function resolve(newValue) {

        if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) {

            var then = newValue.then;

            if (typeof then === 'function') {

                then.call(newValue, resolve);

                return;

            }

        }

        state = 'fulfilled';

        value = newValue;

        setTimeout(function () {

            deferreds.forEach(function (deferred) {

                handle(deferred);

            });

        }, 0);

    }

​

    fn(resolve);

}

​