Promise.js

var PENDING = 1;
var RESOLVED = 2;
var REJECTED = 3;

var PENDING = 1;
var RESOLVED = 2;
var REJECTED = 3;

function Promise(executorFunction) {
    var status = PENDING;
    var success_callbacks = [];
    var error_callbacks = [];

    this.then = function (successCallback) {
        success_callbacks.push(successCallback);
        return this;
    };

    this.catch = function (errorCallback) {
        error_callbacks.push(errorCallback);
        return this;
    };

    var resolve = function (value) {
        if (status !== PENDING) return;

        try {
            success_callbacks.forEach(cb => cb.call(null, value));
            success_callbacks = [];
            status = RESOLVED;
        } catch (e) {
            this.reject(e);
        }
    };

    var reject = function (err) {
        if (status !== PENDING) return;

        error_callbacks.forEach(cb => cb.call(null, err));
        error_callbacks = [];
        status = REJECTED;
    };

    executorFunction && executorFunction.call(null, resolve, reject);
}