1. Null vs Undefined:

In Javascript null is assigned value, which means this value doesn't exist. Without assigning a value of null, you can not get from anywhere by Javascript a Null result.

var assign = null;console.log(assign); //==>  null

‘Null’ result can get only by assigning a variable with ‘null ’ value, which is shown in the example. Javascript Programmer uses this value for different purposes.

In JavaScript Undefined means previously a variable is declared by a programmer but he/she never assigned any value there. We can get Undefined results in many cases.

var taka;
console.log(taka) //==> undifined

There's several ways you can get undefined;

var object = { name: 'Ria Akter' };
console.log(object.cellphone); // ==> undefinedvar array = ['str1', 'str2'];
console.log(array[3]);  // ==> undefinedvar gf = undefined
console.log(gf) // ==> undefined

2. equal (==) vs triple equal (===)

Both are comparison extensions in JavaScript but their comparison method is different. Double equal compare only the “value ” wheres triple equal compare both value and type.

const foo = "test" 
const bar = "test"  

console.log(foo == bar) //true {value same}
console.log(foo === bar) //true {value same type same}const foo = null
const bar = "null"  

console.log(foo == bar) //true {value same}
console.log(foo === bar) //false {value same but type are not}

3 /4/ 5. hoisting, block scope, global variable

We know that there are three types of expression in javascript for declaring a variable: Let, Const & Var.

Hoisting means lifting something. var is the only expression that is hoisted to the upper level from the nested area to the parent.

console.log(x)function parent(){
function child (){
var x = 9
}
}

Here I console.log variable x which is nested in the child function. But it gives you a result escaping error. Because variable x is hoisted here. Javascript hoists the variable “x” to the top but it does not hoist its value. As a result, you get “Undefined” result here.

In the case of Let & Const, the variable doesn't hoist.

console.log(x)function parent(){
function child (){
let x = 9
}
}
// ==> errorconsole.log(x)function parent(){
function child (){
const x = 9
}
}
// ==> error

That means “Let” and “Const” variables create block scope inside curly-bracket{}.

A global variable is declared in the window, outside of the function. We can declare a global variable inside a function by following

function gVariable() {
window.foo = 10 //Global Variable
}

6. Understanding ‘this’

“this” references the object that is currently calling the function. If a function is called with new, this is the object created by the function. If a function called with “call”, “apply”, or “bind”, this refers to the object is passed in the argument. If a function is called with a contextual object, this refers to the contextual object.

7. DOM (document object model):

DOM represents the node tree of all the UI elements present on a web page. If any element on the user interface changes, the DOM updates itself with the new changes.

8. JavaScript callback function:

Callback function is the use of a function as a parameter in another function. We generally used callback function for asynchronous execution & for even handling purposes. Its makes your code clean and DRY (do not repeat yourself).

Here is an example of callback function,

function greetings(string, cbFunction) {
cbFunction(string)
}
function cbFunction(string) {
console.log("Good Morning", string);
}
greetings("Ridwan", cbFunction) // Good Morning Ridwan

here cbFunction is acting as a callback function.

9. bind, call and apply:

Trice are used to collaborate or get a method situated in another objects in javascript. Those are the ways by which we apply a method in an object to another object.

The “bind” method creates a new function that, when it is called, has its “this” keyword set to the provided value.
The “call” method calls a function with a given “this” value and arguments provided individually separated by using comma(,).
The “apply” method calls a function with a given “this” value and arguments provided an array of all of our parameters.

Remember that the purpose of “call” and “apply” methods are same.

10. API (Application Programming Interface):

API provides properties and methods of data structures to the users easily without any “behind the scenes” works. It is kind of readymade food serving!

Top Ten JavaScript Interview Questions for Freshers