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Node.js Backend Engineer Challenge

You can submit all the answers to this assignment in a private, single repository or as a zipped folder, containing markdown and code. If you use GitHub, please share your answers with reedsyapplications.

Answering all the questions to a Production-level standard should take approximately 1 work day.

For coding questions use the latest Node LTS.

1. About you

Tell us about one of your commercial projects with Node.js.

2. Document versioning

Detail how you would store several versioned novels.

Your approach should:

  • show the novel in its current state
  • show the novel at any point in its history
  • show the changes made between two versions
  • prioritize disk space efficiency
  • discuss any trade-offs made, as well as potential mitigations
  • consider any potential domain-specific issues

3. Node.js REST API

Implement a REST API using Express.js that handles Export and Import requests.

The API should expose endpoints to:

  • POST a request for a new Export job. Valid requests should be saved in memory. Invalid requests should return an error. The request must have the following schema:

    {
  bookId: string,
  type: "epub" | "pdf"
}

  • GET a list of Export requests, grouped by their current state (see below).

  • POST a request for a new Import job. Valid requests should be saved in memory. Invalid requests should return an error. The request must have the following schema:

    {
  bookId: string,
  type: "word" | "pdf" | "wattpad" | "evernote",
  url: string
}

  • GET a list of Import requests, grouped by their current state (see below).

Both export and import requests should be created with a pending state, and with a created_at timestamp. An import or export should take the amount of time outlined below. After the specified time, the state should be updated from pending to finished and update an updated_at timestamp.

Job type Processing time (s)
ePub export 10
PDF export 25
import (any) 60

Your solution should:

  • use TypeScript or modern ES features
  • have reasonable test coverage
  • be scalable — this is a small app, but write it as if it will grow into a full, Production-grade server
  • be data store agnostic

4. Operation collision

When multiple users are collaborating on a document, collisions in their edits inevitably occur. Implement a module that can handle basic text update operations, and combine two colliding edits into a single operation.

An operation is described as an array of any combination of three types of edits:

  • { skip: number } to skip characters
  • { insert: string } to insert the given string
  • { delete: number } to delete a number of characters

Implement the following methods:

  • Operation.prototype.combine(operation) Updates the operation by combining it with another colliding operation
  • Operation.combine(op1, op2) Static method that returns a new operation by combining the arguments without mutating them
  • Operation.prototype.apply(string) Applies the operation to the provided argument

For example:

const s = "abcdefg";
const op1 = new Operation([{ skip: 1 }, { insert: "FOO" }]);
const op2 = new Operation([{ skip: 3 }, { insert: "BAR" }]);

expect(op1.apply(s)).to.equal('aFOObcdefg');
expect(op2.apply(s)).to.equal('abcBARdefg');

const combined1 = Operation.combine(op1, op2); // => [{ skip: 1 }, { insert: 'FOO' }, { skip: 2}, { insert: 'BAR' } ]
expect(combined1.apply(s)).to.equal('aFOObcBARdefg');

const combined2 = Operation.combine(op2, op1);
// NB: This expectation is true for this specific case, but not in the general case.
// Can you think of an example where this assertion might not be true?
expect(combined2.apply(s)).to.equal(combined1.apply(s));

Your solution should:

  • use TypeScript or modern ES features
  • have reasonable test coverage
  • explain any assumptions made