// 例5

​

function getUserId() {

    return new Promise(function (resolve, reject) {

        window.setTimeout(function () {

            if (Math.random() >= 0.5) {

                resolve(9876);

            } else {

                reject('not login user');

            }

        });

    });

}

​

getUserId().then(function (id) {

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

}, function (reason) {

    console.error('error msg:' + reason);

});

​

​

function Promise(fn) {

    var state = 'pending',

        value = null,

        deferreds = [];

    this.then = function (onFulfilled, onRejected) {

        return new Promise(function (resolve, reject) {

            handle({

                onFulfilled: onFulfilled || null,

                onRejected: onRejected || null,

                resolve: resolve,

                reject: reject

            });

        });

    };

​

    function handle(deferred) {

        if (state === 'pending') {

            deferreds.push(deferred);

            return;

        }

        var cb = state === 'fulfilled' ? deferred.onFulfilled : deferred.onRejected,

            ret;

        if (cb === null) {

            cb = state === 'fulfilled' ? deferred.resolve : deferred.reject;

            cb(value);

            return;

        }

        ret = cb(value);

        deferred.resolve(ret);

    }

​

    function resolve(newValue) {

        state = 'fulfilled';

        value = newValue;

        finale();

    }

​

    function reject(reason) {

        state = 'rejected';

        value = reason;

        finale();

    }

​

    function finale() {

        deferreds.forEach(function (deferred) {

            handle(deferred);

        });

    }

​

    fn(resolve, reject);

}