JavaScript Interview Questions

Hey amazing reader🤗, welcome back to the series of JavaScript learnings

Today is 9th day if #30daysOfJs.

Today we are going to discuss some of the interview questions asked in JavaScript interviews.

Let's goo 🚀🚀

What are first-class functions?

A programming language is said to have 'First-class functions' if the functions in that language are treated like any other variable.

Hence, the functions in JS are treated as 'first-class citizens', they are treated similar to objects.

So, now according to the definition:

1. Functions in Js can be assigned to any other variable, that means they are treated as values.

2. Function can be passed as an argument to other functions.

3. Function can be a return value of other function.

Let's see all the 3 points with examples.

// 1st point
let func = function(){
    return "This is first-class function"
}

// 2nd point
let anotherFunc = function(func){
    console.log(func() + " 2nd point") 
}

// 3rd point
let thirdFunc = function(){
    return function (params) {
        console.log("This is a function in return")
    }
}

console.log(func())
anotherFunc(func)
thirdFunc()

The output for this is

I hope you've understood it.

We can treat function in JavaScript like any other variable, and hence it is also called as 'First-class citizen'.

Now how a variable can be treated is shown in the above code.

At the end, just remember the three points along with their execution.

What is the difference between a function statement and a function expression?

The difference in syntax is,

function statement -> normal declaration of a function

function expression -> assigning function to a variable, in other words, treating function as a value.

Apart from syntax difference there is a difference of hoisting as well.

Check the image below

Let's see what function hoisting

What are higher-order functions?

A higher-order function is a function that takes a function as an argument or returns a function.

What is function currying?

This is a technique in functional programming, it is transforming functions with multiple arguments into several functions of a single argument in sequence.

What is the difference between the null and the undefined values?

Undefined is a data type, it shows that a variable is declared but not yet initialized with any value.

Whereas null is also a data type that says 'no value' and this can be assigned to the variables.

What is the difference between == and ===?

The == operator is used for loose equality comparisons -> just compares the value

whereas === is used for strict equality comparisons -> comparing value and the data type as well.

What are Arrow functions?

An arrow function is an alternative to write a function, however, function declaration and arrow function have some minor differences.

Arrow function uses arrow instead of the keyword function to declare a function. Let us see both function declaration and arrow function.

What is scope in JavaScript?

Scope in Js determines the visibility or accessibility of variables.

I have already discussed it on day 8.

You can check it out in detail here : Scope in JS

What is NaN in JavaScript?

It stands for 'Not a Number'.

It is returned when the expected input is not a legal number.

To check if something is a NaN or not, we use Number.isNaN()

How to get the type of variable?

Use typeof operator to check the type of a variable

That's it folks for today 🙂

Hope you're enjoying this journey with me 😉.

These were few Interview questions which I have collected from Codedamn's twitter thread.

Here's the link to the thread: Codedamn thread

Take care 🤗

Goodbye 👋