CustomPromise.js

class CustomPromise {
    constructor(executable = (resolve, reject) => {}) {
        this.state = 'pending';
        this.result = undefined;
        this.error = undefined;
        this.thenList = [];
        this.catchList = [];
        this.executable = executable;
        this.execute();
        return this
    }

    execute() {
        try {
            this.executable(this.resolve.bind(this), this.reject.bind(this));
        } catch(err) {
            this.reject(err);
        }
    }

    reject(reason) {
        this.state = 'rejected';
        this.error = reason;
        this.catchList.map(promise => {
            promise.execute();
        })
        this.thenList.map(promise => {
            promise.execute();
        })
    }

    resolve(result) {
        this.result = result;
        if(! (this.result instanceof CustomPromise) && ! (this.result instanceof Promise)) {
            this.state = 'fulfilled';
            this.thenList.map(promise => {
                promise.execute();
            })
        } else {
            this.executable = (resolve, reject) => {
                this.result.then(result => {resolve(result)}).catch(error=> {reject(error)})
            }
            this.execute()
        }
    }

    then(thenHandler) {
        let promise =new CustomPromise((resolve, reject) => {
            if (this.state === 'rejected') throw new Error(this.error)
            if (this.state === 'fulfilled') {
                let result = thenHandler(this.result)
                resolve(result)
            }
        })
        this.thenList.push(promise);
        return promise
    }

    catch(catchHandler) {
        let promise = new CustomPromise((resolve, reject) => {
            if(this.state === 'rejected') {
                catchHandler(this.error)
                resolve()
            }
        })
        this.catchList.push(promise);
        return this
    }
}