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A JavaScript interview question asked at Google

elisabethgross profile image elisabethgross ・3 min read

Hello and welcome back to Code Review, a series of coding interview challenges and career related content released weekly exclusively on Dev.to. I’m Elisabeth and I’ve been a software engineer for about 4+ years now. I’m passionate about sharing my knowledge, and best tips and tricks when it comes to acing that interview and or just leveling up your coding skills. If you want more content and challenges like these, subscribe to the Coderbyte newsletter here. That’s it for stand up - let’s get to challenge solving!

The Challenge

Write a class, EventEmitter that has three methods: on, emit, and removeListener.

  • on("eventName", callbackFn) - a function that takes an eventName and a callbackFn, should save the callbackFn to be called when the event with eventName is emitted.
  • emit("eventName", data) - a function that takes an eventName and data object, should call the callbackFns associated with that event and pass them the data object.
  • removeListener("eventName", callbackFn) - a function that takes eventName and callbackFn, should remove that callbackFn from the event.

For example:

let superbowl = new EventEmitter()

const cheer = function (eventData) {
  console.log('RAAAAAHHHH!!!! Go ' + eventData.scoringTeam)

const jeer = function (eventData) {
  console.log('BOOOOOO ' + eventData.scoringTeam)

superbowl.on('touchdown', cheer)
superbowl.on('touchdown', jeer)

superbowl.emit('touchdown', { scoringTeam: 'Patriots' }) // Both cheer and jeer should have been called with data

superbowl.removeListener('touchdown', jeer)

superbowl.emit('touchdown', { scoringTeam: 'Seahawks' }); // Only cheer should have been called

The solution:

This is a great opportunity to use ES6 classes. In case you haven’t used them before, check out their syntax here. We can start with a basic structure for the class EventEmitter and initialize it with an object events that we will use to track our events.

class EventEmitter {
  constructor () {
    this.events = {}


Next we can start working on our methods. First up is on. Here is the code for that:

on (eventName, callbackFn) {
  if (!this.events[eventName])  {
    this.events[eventName] = []

Because functions are first class objects in javascript, which basically means they can be stored in a variable, an object, or an array, we can just push the callback function to an array stored at the key eventName in our events object.


Now, for our emit function.

  emit (eventName, eventData) {
    if (!this.events[eventName]) return
    this.events[eventName].forEach(fn => fn(eventData))  

This solution takes advantage of what is called closure in javascript. If you are coding in Javascript in your interview, understanding closure can be vital. A closure is essentially when a function has references to its surrounding state or its lexical environment. You can also think of this as a closure allowing you access to an outer function’s scope from inside an inner function. Using global variables is a great simple example of closure.

Here’s another great example of using closure to track how many times a function was called.

function tracker (fn) {
  let numTimesCalled = 0
  return function () {
    console.log('I was called', numTimesCalled)
    return fn()

function hello () {

const trackedHello = tracker(hello)

The inner function returned in tracker closes over the variable numTimesCalled and maintains a reference to it for the life of the trackedHello function. Cool stuff huh??


The removeListener method is probably the easiest of the three. Here is a solution -

removeListener (eventName, callbackFn) {
  const idx = this.events[eventName].indexOf(callbackFn)
  if (idx === -1) return
  this.events[eventName].splice(idx, 1)

And that’s the class! Pun intended :) Seeing if you can implement methods that are part of the language is a great way to practice for interviews. See you all next week!

Posted on by:

elisabethgross profile



I am a full stack engineer, passionate about solving complex problems and collaborating with driven teams! I have 4+ years of experience working at small to mid sized startups.


Coderbyte is a web application built to help you practice programming and improve your coding skills.


Editor guide

That looks like an unnecessarily complex and overengineered solution.

Google will love you.


I know this is an unfashionable view these days, but to me this is a perfect example of why you need to be careful with Typescript. This is theoretically type safe, but it's harder to read and maintain, and do you really need this level of type safety in a simple event system like this?
It could probably be done more simply in TS than this, but a bit like Java sometimes it's as if the language itself encourages overengineering.


I liked this problem! Didn't read the solution so I could practice it myself. Here's what I came up with:

class EventEmitter {
  constructor() {
    this.events = {};

  on(event, listener) {
    if (!this.events[event]) {
      this.events[event] = [];

  emit(event, data) {
    this.events[event].forEach(listener => {

  removeListener(event, listenerToRemove) {
    // Return if we never registered any listeners for this event
    if (!this.events[event]) return;
    this.events[event] = this.events[event].filter(
      listener => listener !== listenerToRemove

Edit: Okay, having read the solution, it seems like we did a lot of things similarly! My removeListener may not be too performant, though, since it replaces the whole array.

Edit2: I believe there's a bug in your removeListener that my code accounted for. If you pass in an event that we never listened to, then attempting to execute this.events[event].indexOf(...) will throw an error because this.events[event] will be undefined.


That's not so different than a typical implementation:




I wouldn't do that, mainly because it's unnecessarily slow for anything but the first listener. My version (and the solution) just pushes the new listener to the back of the array, which is an O(1) operation. Your version uses the slice operator and creates a new array, and assigns that to the old one. That's O(n).


Regardless of whatever you two decide is a better answer, its always nice to see other ways of solving the same problem! Play nicely :)


Good point that will definitely be a bug! Nice catch!


That's a good question, and the solution is really clear👍

I'm not sure I'd call the emit function a closure, it's not returning a function, and it's not being run outside it's lexical scope (although the callbacks are closures themselves I guess)


Not the emit function - the event callbacks themself close over the data object and gain access to that themselves!


I think we can't call any function a closure. We just can say that some function uses a closure. emit function actually uses closure here (inside forEach callback). I can be wrong)

There's a lot of visual clutter, and lots of helper functions that aren't really necessary (toArray, equals, and notEquals). In particular, there's no point in defining wrappers like this:

const equals = <T>(a: T) => <R extends T>(b: R): boolean => a === b;

Typing in this case is also overkill, imo, except arguably for the data object that's getting passed in to the listeners.

Again, overengineering. Google engineers do this a lot. You'll know when you try to implement a simple feature in Android and realize that the brilliant minds at Google thought it would be creative to have you create 3-4 boilerplate classes just so you can display a simple list of items.

There was no need for this. It's incredible the lengths to which you went to complicate a simple equality expression—creating not just notEqual but also its binary inverse, equal, which doesn't actually get used except in the definition of, you guessed it, notEqual 😂.

Unnecessary abstraction, followed by an essay justifying it.

The pragmatic thing to do is to first write the code that gets the job done, and only then to consider how the code can be improved. equal and notEqual are perfect examples of abstractions that serve no meaningful purpose in your code. They directly substitute one expression with another, and are only used once. That's not grounds for refactoring.

I'm totally willing to accept that you are correct if you can supply some rigorous model for differentiating necessary from unnecessary abstractions.

I don't really think one has to define a "rigorous model." In my view, it comes down to common sense, and answering one question: Is it worth investing additional time to create an abstraction if something simpler would suffice, while retaining code clarity?

Breaking that down further:

  1. Does this abstraction make my code cleaner?
  2. Does this abstraction reduce the amount of typing I have to do?

Your code fails on both fronts. My eyes bleed when I see code like this:

const toArray = <T>(arr?: T[] | null): T[] => (Array.isArray(arr) ? arr : []);

The interview question asks for an implementation of an EventEmitter class. What you are proposing is that I, with my limited time and resources:

  1. Litter my code with an alphabet soup of Ts, Rs, as, and bs, and introduce unnecessary typing information.
  2. Create a function named equal that serves no real purpose elsewhere in the codebase, except to be used in its binary inverse, notEquals.
  3. Create a curried function notEquals that has all the blessings of type safety.
  4. Use that function with notEquals<unknown> (obligatory lol).

I'm not an interviewer. But if I were one, I can't imagine I'd be too happy with you wasting my time (and yours) by writing unnecessary code.

I love TypeScript as much as the next dev. I used it during my internship and for a pretty involved web app project in my final semester of school. But this... This is overkill, and more of a pain than a blessing.

Its nice that you were able to make this a productive conversation full of exchanging knowledge. You go team!


while I like the content, Google has not been a good place for developers for a while and we should stop idolizing the "anything related to google must be good" label


Couldn't agree more. Idolizing any company is a recipe for disappointment. However, they are KNOWN for the caliber of interview questions they ask so its really nice to see some real questions they've asked in the past!


There's a couple of things that bother me about this solution, both of them in emit.

First, there's no error handling, so a called function could break the forEach with a throw.

The second is a little more subtle, and I'm not sure if, or how it should be fixed. The object is mutable, and one of the callbacks could change it. Is it canonical to not worry about this? How would you go about fixing it? Deep copy? Freezing? Since it's a parameter, not sure the best way.


Making the eventData immutable is probably not the job of the EventEmitter

The Node.js EventEmitter doesn't alter the data in its emit implementation


That's why I don't know the solution to this. In C++, passing along const references solves it from anything but something very intentional. If it's not immutable (or rather, some callback decides to mutate it) you're going to run into all sorts of potential bugs. Things can change based on the order of the callbacks and on the implementation of the emitter. That just really grates on me and feels wrong.

I wonder if that's part of the point of the question. When I did C++ interviews, one of my standard questions was "implement memcpy". I was more interested in how they handled edge cases than the actual copying.

It's dangerous to assume that the theoretical time complexity of any algorithm is going to be the same as the empirical time complexity.

Except in this case, array assignment is definitely slower than pushing a single element to the back of an existing array.

Also, it seems you didn't actually read the post that you linked:

With this optimization, the performance test shows that [...array, item] can perform at least twice faster than [item, ...array].

That's not what we're comparing here. We're comparing [...array, item] and array.push(item). The latter is indisputably faster.


I would say that the implementation is straight forward, but my brain focused on the events member and how it is being used. I have seen this pattern many times and I am surprised there are no common specialized classes for it.

So, first of all, without changing any types, I would optimize the on (and similarly emit) methods like this:

on (eventName, callbackFn) {
  let list = this.events[eventName];
  if (!list) {
    list = [];
    this.events[eventName] = list;

It's a minor improvement for small numbers, but it hurts me to see repeated key access to the same reference.

Second of all, why not write first a class that we could call Lookup to handle all those string indexed lists?

class Lookup {
    constructor() {
        this._items=new Map();

    add (key,obj) {
        let list = this._items.get(key);
        if (!list) {

    remove (key, obj) {
        const list = this._items.get(key);
        if (!list) return;
        const index = list.indexOf(obj);
        if (index>=0) {

    get(key) {
        // can return an empty array, too, but one could then try to push items into it
        return this._items.get(key) || function*(){};

    clear(key) {
        if (typeof key === 'undefined') {

Now, if events is a Lookup class, the code is cleaner and safer:

on (eventName, callbackFn) {
  this.events.add(eventName, callbackFn);
emit (eventName, eventData) {
  for (const fn of this.events.get(eventName)) {
removeListener (eventName, callbackFn) {
  this.events.remove(eventName, callbackFn);

In fact, most of the code in your EventEmitter class is actually Lookup code. One could add to the Lookup class a forEach method and you wouldn't even need an EventEmitter class:

forEach(key,func) {
  for (const item of this.get(key)) {

Also, being all modern and not ending statements in semicolon feels obnoxious to me. It's probably me being me and old, but I had to say it.


I'm still trying to understand the need for equals and notEqual. You already have the === operator, which checks both type and value equality. Why wrap that in a function?


how about doing a filter for removing a listener i.e filtering in objects which need not be removed.


Sorry to poop the party, but I hope google asks better questions in interviews for their sake.


Anyone smell that..? ;) Sometimes you get lucky with an "easier" problem - not the worst thing in the world!


Very good post. Thanks.


You could also have used a Set, assuming that functions are added only one.


I see a lot of comments related to ts, but the article dowsnt seem to use ts, did it get edited?


Hi, Elisabeth.

I want to share my solution to your series in the Korean community.
Are there any rules I must follow?


Nice, concise and readable answer to a common JS problem that most of us have ended up implementing in one way or another over the years.


Holy shit. I ask the same question in my interviews. Its create a simple EventBus with fire,listen and remove methods 😅