Basic promise implementation

Custom-Promise.js

function CustomPromise(fn) {
    this.chain = [];
    this.onResolve = this.onResolve.bind(this);
    this.onReject = this.onReject.bind(this);
    fn(this.onResolve, this.onReject);
}

CustomPromise.prototype.then = function (fn) {
    this.chain.push(fn);
    return this;
}

CustomPromise.prototype.catch = function (fn) {
    this.errorFn = fn;
    return this;
}

CustomPromise.prototype.onResolve = function (resolvedValue) {
    let storedValue = resolvedValue;
    for (let i = 0; i < this.chain.length; i++) {
        const fn = this.chain[i];
        try {
            storedValue = fn(storedValue);
        } catch (err) {
            this.onReject(err);
        }
    }
}