Simple polyfill for JavaScript Promise class

CustomPromise.js

function CustomPromise (executor) {
    let onResolve;
    let onReject;
    let isFulfilled = false;
    let isRejected = false;
    let isCalled = false; // indicates callback has been called
    let value; let error;
    function resolve(val) {
        isFulfilled = true;
        value = val;
        if (typeof onResolve === 'function' && ! isCalled) {
            onResolve (val);
            isCalled = true;
        }
    }
    function reject(err) {
        isRejected = true;
        error = err;
        if (typeof onReject === 'function' && !isCalled) {
            onReject(err);
            isCalled = true;
        }
    }

    this.then = function (thenHandler) {
        onResolve = thenHandler;
        if (isCalled && isFulfilled) {
            onResolve(value);
            isCalled = true;
        }
        return this;
    };

    this.catch = function (catchHandler) {
        onReject = catchHandler;
        if (!isCalled && isRejected) {
            onReject(error);
            isCalled = true;
        }
        return this;
    }
    executor( resolve, reject);
}

MyPromise.resolve = (val) => {
    return new MyPromise(function executor (resolve, reject) {
        resolve(val);
    })
}
MyPromise.reject = (reason) => {
    return new MyPromise(function executor(resolve, reject) {
    reject(reason);
    });
}
MyPromise.all = function (promises) {
    return new MyPromise( function executor (resolve, reject) {
        let cnt = 0;
        let res = [];
        if (promises. length === 0) {
            resolve(promises);
            return;
        }
        for (let i = 0; i < promises. length; i++) {
            promises[i].then((val) => {
                done (val, i);
            }).catch((err) => reject(err));
        }
        function done(val, i) {
            res[i] = val;
            ++cht;
            if (promises. length === cnt) {
                resolve(res);
            }
        }
    });
}