Simple Promise Impl to help understand promises

iPromise.js

// simplified promise implementation for learning purposes
// https://levelup.gitconnected.com/understand-javascript-promises-by-building-a-promise-from-scratch-84c0fd855720

class MyPromise {
	constructor(promiseCallback) {
		this.promiseChain = []
		this.handleError = () => {}

		// bind this as onResolve and onReject callbacks are passed into the promise callback
		this.onResolve = this.onResolve.bind(this)
		this.onReject  = this.onReject.bind(this)

		promiseCallback(this.onResolve, this.onReject)
	}

	then(onResolve) {
		this.promiseChain.push(onResolve)
		return this
	}

	catch(handleError) {
		this.handleError = handleError
		return this
	}

	// in reality you should not loop through the promise chain immediately but only on the resolved values
	// in this implementation you cannot return promises in the then chain
	onResolve(value) {
		let storedValue = value

		try {
			this.promiseChain.forEach((nextFunc) => {
				storedValue = nextFunc(storedValue)
			})
		} catch(error) {
			this.promiseChain = []
			this.onReject(error)
		}
	}

	onReject(error) {
		this.handleError(error)
	}
}

// contrived fetch implementation for the purposes of the example
const myFetch = () => {
	return new MyPromise((resolve, reject) => {
		setTimeout(() => {
			const apiResponse = mockApiBackend()

			if (apiResponse.statusCode >= 400) {
				reject(apiResponse)
			} else {
				resolve(apiResponse.data)
			}
		}, 5000)
	})
}

const mockApiBackend = () => {
	const user = {
		username: 'gsrai',
		age: 24,
		yearOfBirth: 1994
	}

	if (Math.random() > 0.05) {
		return {
			data: user,
			statusCode: 200
		}
	} else {
		return {
			statusCode: 404,
			message: 'Could not find User',
			error: 'Not Found'
		}
	}
}

function main() {
	myFetch().then(user => {
		console.log('first then(), ', user.username)
		return user
	}).then((user) => {
    		console.log(`User ${user.username}'s favorite number is ${user.age}`)
  		return user
  	}).then((user) => {
    		console.log('The previous .then() told you the age')
    		return user.yearOfBirth
  	}).then((year) => {
    		console.log(`Their year of birth is ${year}`)
	})
  	.then(() => {
    		console.log('This is the last then()')
  	})
  	.catch((error) => {
    		console.log(error.message)
  	})
}

main()