JavaScript Interview Questions that made me think — Do I really know JavaScript? — Part 3

Javascript tricky interview questions

Javascript is full of surprises, Here I am sharing few question which seems very interesting.

What will be printed in the console?

.

.

.

Error will be thrwon in the console:

Uncaught SyntaxError: identifier starts immediately after numeric literal

This is due to language grammar limitation.
The dot(.) character could mean different things in JS. It can be seen as the member operator, or a decimal, depending on its placement.

In order to make above code work we have to either use parenthesis or an additional dot to make the expression valid.

21..toString(); // "21";

(21).toString(); // "21";

What will be printed in the console?

.

.

.

.

4520, 92

So First operation is obivious, whenever we do add(+) operation with string,
JS will concatanate the string.

But for ++ what went different?

It's the implementation of ++ and ++, both of these first convert the argument
to Number and then do a ++.

function plusplus(number) {
    const num = Number(number);
    num = num+1;
    return num';
}

What will be printed in the console?

.

.

.

.

true

Let's break it down.

Whenever we do a double equality check, JS engine will try to convert both 
values in Number.

Number([]) == Number(![]);
0 == Number(false); // ![] will give false
0 == 0 // true

What will be returned from the function?

.

.

.

.

90000

Here if we break down this as,

const res = cosmic = 90000;
return res;

// Undeclared cosmic will be added to global scrope and 90000 will be returned.

What will be logged in the console?

.

.

.

.

Water Added
Uncaught TypeError: addSugar is not a function

What will be printed in the console?

.

.

.

.

3, Hello

this is pointing to array itself, and the length of array is 3.

What will be logged in the console?

.

.

.

.

[ 10, 20, 3, 7 ]

By default, sort method performs sorting by converting the numbers into strings. Then it compares their sequences of UTF-16 code unit values.

To sort the numbers by their numeric value, we’ll need to pass a compareFn to the sort() method:

[ 10, 20, 3, 7 ].sort((a, b) => a - b); //[ 3, 7, 10, 20]

In case you have any doubt or want to understand any concept related to web development, book as session with me on topmate. https://topmate.io/frontendmaster

Here are the other parts of this series:

JavaScript Interview Questions that made me think — Do I really know JavaScript? — Part 4

JavaScript Interview Questions that made me think — Do I really know JavaScript? — Part 2

JavaScript Interview Questions that made me think — Do I really know JavaScript.