Back-end Coding Challenge

Welcome to this back-end Coding Challenge! We will ask you to complete the following exercice as part of our Interview process. It aims at assessing your coding skills proficiency, and your problem-solving mindset. It should take you no more than 2h.

You can submit this article as a public GitHub repository with the exercice and a short README explaining how to run it. You can write your code in Go (preferred), JavaScript or Python.

ℹ️

Clean and concise code is expected.

Let User and Action be two sets of records we store in our database. User contains basic information about our product’s users, and an Action is created when a user interacts with some features of our product, so we can keep an eye on our user’s behavior and improve our product accordingly.

Here’s how these two record types are structured:

User: {
	id: int
	name: string
	createdAt: date
}

Action: {
	id: int
	type: string
	userId: int       // The ID of the User who performed this action
	targetUser: int   // Supplied when "REFER_USER" action type
	createdAt: date
}

Provided the following two JSON files containing a database sample for this exercice, please answer the questions below. Your work should be organized as a very simple web server, so we can get the answer of each question by querying the relevant API endpoint. There is no need to implement the database layer, reading the file in memory at startup is sufficient.

users.json103.5KB

actions.json3058.8KB

Q1) Write an API endpoint to fetch a User given its ID

Example response:

{
	id: "1234"
	name: "John Doe"
	createdAt: "2022-04-14T11:12:22.758Z"
}

Q2) Write an API endpoint to get the total number of actions of a User given its ID

Example response:

{
	count: 100
}

Q3) Write an API endpoint to get the break-down of all possible next actions given an action Type

Users perform different actions throughout the day when using our product. Right after doing an action A, users could perform action B, C, or A again for instance. Write an endpoint that gives a probability break-down of what actions our users typically do after doing action A, based on the entire Actions database.

Example response:

{
	ADD_TO_CRM: 0.70
	REFER_USER: 0.20
	VIEW_CONVERSATION: 0.10
}

Q4) Write an efficient API endpoint to get the “Referral Index” of all the Users. Discuss complexity.

Users have the ability to refer other users (i.e. inviting them to use the product as well). When doing so, an activity with type REFER_USER is created by the existing user, and the ID of the new invited user is stored in the targetUser attribute.

We can therefore compute the Referral Index of a given user, as the total number of individual users invited directly on indirectly by this user (users invited by the given user can also invite new users, etc...)

Let’s assume a user can be invited only ONCE.

Example response:

{
	1: 3 // UserID: Referral index
	2: 0
	3: 7
	...
}