Ten JavaScript Interview Questions You have to Know

1.Truthy or Falsy

javaScript determines some values which can true or false.

How to memories it?

· There are all the variables that has a value except 0(zero) is true
· There are all strings are true except “empty string”.

Falsy:

False, 0, “”,undefined,null, NAN

Truthy:

‘0’, ” ”, [],{},’false’

How to check it?

const variableName = undefined;
if(variableName){
console.log(“this is true”);
}
else{
console.log(“this is false”);
}
Output:
this is false

Note: “0” (zero) and ‘ ’(white space) are true

2.Null vs Undefined

Null: Null means empty value that assigns before but that value gets away from the variable.

Undefined:

We can get undefined some ways that we discuss below:

· If we not assign a value in variable
· Forget to return something from function
· Don’t assign all the parameters of the function
· Don’t assign a value in the object but read that
· Don’t assign a value in the array but read that
· If the variable consist a value of undefined

3.(==) vs (===)

This is a simple but important topic, What the difference between double and triple equal

Double equal: it just read the value of the property

const variableName = 14;
if (variableName == “14”) {
console.log(“this is true”);
}
else {
console.log(“this is false”);
}
Output:
this is true

Triple equal: it read the value of the property but also read the type of that property

const variableName = 14;
if (variableName === “14”) {
console.log(“this is true”);
}
else {
console.log(“this is false”);
}
Output:
this is false

4.what is hoisting?

A variable can be used before it declares this is called hoisting.

console.log(name);
var name = “naim”
output:
undefined
var name = “naim”
console.log(name);
output:
naim
let name = “naim”
console.log(name);
output:
naim
console.log(name);
let name = “naim” //let or const
output:
error

at first, we declare a variable using var and output of it undefined, second and thirdly we get a result but at the last, we get an error, var make a global scope, where let or const make a block scope, for this reason, this happened.

5.what is closure?

If we call or return a function inside a function , these functions create a close environment between them this situation is called as closure or encapsulation or private variable,

const result1 = counter();
const result2 = counter()
console.log(result1());
console.log(result1());
console.log(result2());
console.log(result2());
console.log(result2());
output:
1
2
1
2
3

6. var vs let vs const

We know in javascript a variable can declare by var or let or const. In 2015 when ES6 arrive we first knew about let and const . var create a global scope but let and const create a block scope. Var or let can be updated and reassign its value but in const we can not reassign values.

7.how to calculate factorial of a number using for loop?

Firstly we declare a variable named number and initial value of the num of factorial equal to zero,

num! =(num)*(num-1)*(num-2)…….*1

the we apply a for loop from 1 to index of the num ,and multiply the factorial by i ,

and finally we got our answer

const num = 5;
let factorial = 1;
for (let i = 1; i <= num; i++) {
factorial = factorial * i;
}
console.log(factorial);
output:
120

8.is the number prime or not?

Definition of a prime number is exactly not divisible by all number except 1 and the same number.

19 is a prime number because it is only exactly divisible by 1 and 19.

Code of the program:

function isPrime(number) {
for (let i = 2; i < number-1; i++) {
if(number%i===0){
return “its not a prime number”;
}
else{
return “its a prime number”;
}
}
}
const result = isPrime(19)
console.log(result);
output:
its not a prime number

9.how to find largest value of an array?

Let the maximum value is the first element of the array that index is 0(zero).

Now apply a for loop from first to last index of the array, and compare the element value with max value

Code of the program:

const array = [10, 20, 30, 40];
let max = array[0];
for (let i = 0; i < array.length; i++) {
const element = array[i];
if(element>max){
max=element;
}
}
console.log(max);
output:
40

10.how to remove duplicate item from an array?

Let a uniqueNumber equal to [] (empty array), then apply a for loop from first to end of the index of the array, and check the index number of unqueNumber, if index number is -1 then push the value into unqueNumer.

Code of the program:

const array = [10, 20,30,14,15,20,40 ,30, 40];
let uniqueNumber = [];
for (let i = 0; i < array.length; i++) {
const element = array[i];
const index = uniqueNumber.indexOf(element)
if(index==-1){
uniqueNumber.push(element)
}
}
console.log(uniqueNumber);
output:
[ 10, 20, 30, 14, 15, 40 ]