Abdullah Mamun
2 min readMay 8, 2021


What is JavaScript?

JavaScript is a dynamic programming language. You can also called it Scripting language. That’s used for web development, in websites. Using JavaScript you can implement dynamic features on web pages, that cannot be done with html and CSS.

What is undefined?

Undefined: Undefined means a variable has been, but not define. more easily, we declare a variable, but not set to any data, except undefined. There is a lot of way, we can get undefine in JavaScript.


let hello;

console.log(hello); // undefined

we can set a variable to undefined.

let test = undefined;

console.log(test) //undefined

when we looking up a non-existing properties in object, it also give us undefined.

const person = { name: 'samad karim' age: 25, address: 'dhaka, bangladesh'}

console.log(person.phone) //undefined

What is Null?

Null: Null is empty or non-existing. null is a self-identity data types in javascript. we found it only, when we are set a variable to null.


const test = null;

console.log(test) //null

What is double equal (==) and triple equal (===)?

Double Equal (==): When we testing double equal in JavaScript. That’s means we are checking only value, after covert them a common type.


const a = 50;

const b = '50'

console.log(a == b) // true

Triple Equal (===): When we using triple equal in JavaScript, we are checking with strict equality. That’s means both of value and type have to be the same.


console.log( 100 === '100') // false

console.log( 500 === 500) // true

console.log( 'dhaka' === 'bangladesh') // false

console.log( 'bangladesh' === 'bangladesh') // true

What is Scope?

Scope: Scope determines the accessibility of variables. more easily, scope set to us a variable we declare before where and how we access them.

What is block Scope?

Block scope: when we create a function, that’s create a new scope for us. we can not access any variable that’s declare inside of function from outside of function. it’s called block scope.

What is Global Scope?

Global Scope: A variable that declare in outside of block scope it’s become a global scope. we can access it where we want it in the file.

What is dom?

Dom (document object model) is a programming interfaces for HTML and XML. It represent the page, so we can change structure, style, content using this program. Dom represent the document with node and objects. That’s programming language can easily connect with the page.

What is Map, Filter and Find?

Map: Using map we can do anything with array item and return a new array.

Filter: We can filter a specific value from an array. filter give us all matched item, that’s met the condition. filter give us all item in new array.

Find: If we need to get a specific item from an array. we can use find, that’s give us a single result.

What’s difference between var, let, const?

var : var is function scope and global scope.

let: let is block scope.

const: const is a block, cannot be reassigned multiple time.