custom promise. is likely as the window.Promise

customPromise.js

let once = function (fn) {
    let used = false;
    return function () {
        if (!used) {
            fn.apply(null, arguments);
            used = true;
        }
    }
};


let customPromise = function (fn) {
    this.ex = null;
    let valStack = [];
    let exStack = [];
    let thenHandlerStack = [];
    let catchHandlerStack = [];

    let resolve = function (data) {
        valStack.push(data);
        isExistValueThenDO();
    };
    let reject = function (ex) {
        exStack.push(ex);
        isExistExceptionThenDO();
    };

    let isExistValueThenDO = function () {
        if (valStack.length > 0 && thenHandlerStack.length > 0) {
            try {
                let res = thenHandlerStack.pop().call(null, valStack.pop());
                dealThenOrCatchResponse(res);
            } catch (ex) {
                reject(ex);
            }
        }
    };
    let isExistExceptionThenDO = function () {
        if (exStack.length > 0 && catchHandlerStack.length > 0) {
            try {
                let res = catchHandlerStack.pop().call(null, exStack.pop());
                dealThenOrCatchResponse(res);
            } catch (ex) {
                reject(ex);
            }
        }
    };

    let dealThenOrCatchResponse = function (res) {
        if (res instanceof Error) {
            exStack.push(res);
        } else {
            valStack.push(res);
        }
    };

    this.then = function (fn) {
        thenHandlerStack.push(fn);
        isExistValueThenDO();
        return this;
    };



    this.catch = function (fn) {
        catchHandlerStack.push(fn);
        isExistExceptionThenDO();
        return this;
    };


    try {
        fn.call(null, once(resolve), once(reject));
    } catch (ex) {
        reject(ex);
    }
};


let promise = new customPromise((resolve, reject) => {
    resolve(0);
    setTimeout(() => resolve(1), 1000);
});

promise.then(function (val) {
    console.log(val);
    return "from then 1";
}).then(function (data) {
    console.log(data);
}).then((data) => {
    console.log(data);
    return "me is not undefined"
}).then(data => {
    console.log(data);
    dfasdf()
}).catch(ex => {
    console.log(ex);
    return "exception"
}).then(function (data) {
    console.log(data);
});