雏形

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) {
        setTimeout(function() {
            callbacks.forEach(function (callback) {
                callback(value);
            });
        }, 0)
    }

    fn(resolve);
}

剩下的就是加入状态啦，回调支持reject啦

注意resolve中的setTimeout，是为了解决resolve()发生在then注册回调之前，如：

const p = new Promise(resolve => {
    resolve("finished");
});

p.then(result => console.log(`result is ${result}`))