My top stressing things in software development are:
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 that you will be asked for 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.
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!
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
.
b
is an accidently 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; // => 'undefined'
typeof window.b; // => 'number'
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
.
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.
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 {
:
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++) {
// does nothing
}
{
// a simple block
numbers.push(i + 1);
}
numbers; // => [5]
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]
.
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 put high attention to detail.”
Fortunately, I didn’t end up working for that company.
I’ll leave the conclusion up to you.
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); // => undefined
return;
inside the function makes it return undefined
.
So arrayFromValue(10)
evaluates to undefined
.
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
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()
.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:
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!
What’s the result of the equality check?
0.1 + 0.2 === 0.3 // => ???
First, let’s look at the value of 0.1 + 0.2
:
0.1 + 0.2; // => 0.30000000000000004
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.
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.
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
.
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.
#javascript #interview-questions