7 Simple but Tricky JavaScript Interview Questions

My top stressing things in software development are:

  1. The coding interview
  2. A toxic manager or teammate

Not JavaScript, this, CSS, Internet Explorer, but the 2 above.

If you’re qualifying as Senior Developer that involves JavaScript, there’s a good chance to be asked tricky questions during the coding interview.

I know it’s unfair. Some unknown people put you on the edge to see what you’re made of. It’s an unpleasant experience.

Job interview

What can you do?

Follow the advice: “Practice makes perfect”. By investing enough time, better regularly, to deeply understand JavaScript will improve your coding, and as a positive side effect, interviewing skills.

In this post, you will find 7 at first sight simple, but tricky JavaScript interview questions.

While at first the questions might seem random, they try to hook into important concepts of JavaScript. So you better practice them before your next interview!

1. Accidental global variable


To what evaluates typeof a and typeof b in the following snippet:

function foo() { let a = b = 0; a++; return a;
} foo();
typeof a; typeof b; 


Let’s take a closer look at the line 2: let a = b = 0. This statement indeed declares a local variable a. However, it does declare a global variable b.

There is no variable b declared neither in the foo() scope or global scope. So JavaScript interprets b = 0 expression as window.b = 0.

Accidental global variables JavaScript

b is an accidentally created global variable.

In a browser, the above code snippet is equivalent to:

function foo() {
 let a; window.b = 0; a = window.b; a++; return a;
} foo();
typeof a; typeof window.b; 

typeof a is 'undefined'. The variable a is declared only within foo() scope and is not available in the outside scope.

typeof b evaluates to 'number'. b is a global variable with the value 0.

2. Array length property


What is the value of clothes[0]:

const clothes = ['jacket', 't-shirt'];
clothes.length = 0; clothes[0]; 


length property of the array object has a special behavior:

Reducing the value of the length property has the side-effect of deleting own array elements whose array index is between the old and new length values.

Because of this length behavior, when JavaScript executes clothes.length = 0, all the items of the array clothes are deleted.

clothes[0] is undefined, because clothes array was emptied.

3. Eagle eye test


What is the content of numbers array:

const length = 4;
const numbers = [];
for (var i = 0; i < length; i++);{ numbers.push(i + 1);
} numbers; 


Let’s take a closer look at the semicolon ; that appears right before the opening curly brace {:

The null statement effect

It’s easy to overlook this semicolon, while it creates a null statement. The null statement is an empty statement that does nothing.

for() iterates 4 times over the null statement (that does nothing), ignoring the block that actually pushes items to array: { numbers.push(i + 1); }.

The above code is equivalent to the following:

const length = 4;
const numbers = [];
var i;
for (i = 0; i < length; i++) { }
{ numbers.push(i + 1);
} numbers; 

for() increments i variable until 4. Then JavaScript enters one time the block { numbers.push(i + 1); }, pushing 4 + 1 to numbers array.

Thus numbers is [5].

My story behind this question

Long time ago, when I was interviewing for my first job, I was asked this question.

For the interview I was given 20 coding questions to answer within 1 hour limit. The null statement question was on the list.

When solving the question, being in a rush, I didn’t see the comma ; right before the curly brace {. So I answered incorrectly [1, 2, 3, 4].

I was slightly disappointed because of such unfair tricks. I asked the interviewer what is the reason behind tricks like that? The interviewer replied:

“Because we need people that have good attention to detail.”

Fortunately, I didn’t end up working for that company.

I’ll leave the conclusion up to you.

4. Automatic semicolon insertion


What value is returned by arrayFromValue()?

function arrayFromValue(item) { return [items];
} arrayFromValue(10); 


It’s easy to miss the new line between the return keyword and [items] expression.

The newline makes the JavaScript automatically insert a semicolon between return and [items] expression.

Here’s an equivalent code with the semicolon inserted after return:

function arrayFromValue(item) {
 return; [items];
} arrayFromValue(10); 

return; inside the function makes it return undefined.

So arrayFromValue(10) evaluates to undefined.

Follow this section to read more about automatic semicolon insertion.

5. The classic question: tricky closure


What will output to console the following script:

let i;
for (i = 0; i < 3; i++) { const log = () => {
 console.log(i); } setTimeout(log, 100);


If you didn’t hear about this tricky question before, most likely your answer is 0, 1 and 2, which is incorrect. When I first tried to solve it, this was my answer too!

There are 2 phases behind executing this snippet.

Phase 1

  1. for() iterating 3 times. During each iteration a new function log() is created, which captures the variable i. Then setTimout() schedules an execution of log().
  2. When for() cycle completes, i variable has value 3.

log() is a closure that captures the variable i, which is defined in the outside scope of for() cycle. It’s important to understand that the closure captures i variable lexically.

Phase 2

The second phase happens after 100ms:

  1. The 3 scheduled log() callbacks are called by setTimeout(). log() reads the current value of variable i, which is 3, and logs to console 3.

That’s why the output to the console is 3, 3 and 3.

Do you know how to fix the snippet to log 0, 1, and 3? Please write your solution in a comment below!

6. Floating point math


What’s the result of the equality check?


First, let’s look at the value of 0.1 + 0.2:

The sum of 0.1 and 0.2 numbers is not exactly 0.3, but slightly above 0.3.

Due to how floating point numbers are encoded in binary, operations like addition of floating point numbers are subject to rounding errors.

Simply put, comparing floats directly is not precise.

Thus 0.1 + 0.2 === 0.3 is false.

Check 0.30000000000000004.com for more information.

7. Hoisting


What happens if you access myVar and myConst before declaration?

myVar; myConst; 
var myVar = 'value';
const myConst = 3.14;


Hoisting and temporal dead zone are 2 important concepts that influence the lifecycle of JavaScript variables.

Temporal dead zone and hoisting in JavaScript

Accessing myVar before declaration evaluates to undefined. A hoisted var variable, before its initialization, has an undefined value.

However, accessing myConst before the declaration line throws a ReferenceError. const variables are in a temporal dead zone until the declaration line const myConst = 3.14.

Follow the guide JavaScript Variables Hoisting in Details to get a good grasp on hoisting.

8. Key takeaways

You can think that some of the questions are useless for interviewing. I have the same feeling, especially regarding the eagle eye test. Still, they could be asked.

Anyways, most of these questions can truly assess if you are seasoned in JavaScript. If you had difficulties to answer some while reading the post, it’s a good indicator of what you must study next!

Is it fair to ask tricky questions during the interview? Let me know your opinion.