Variables And Data Types JavaScript Interview Questions

 

1. Explain global and local variables in JavaScript?

Local Variables:

Local variables are variables that are defined within functions. They have local scope, which means that they can only be used within the functions that define them.

Global Variables:

Global variables are variables that are defined outside of functions. These variables have global scope, so they can be used in any function without passing as arguments.

2. What are the differences between let and const variables in JS?

let	const
let  is used for variable declaration. Variables declared with the  let  can change it's values	Variables declared with the  const  maintain constant values.  const  declarations share some similarities with  let  declarations
let  can be updated but not re-declared	const  cannot be updated or re-declared
let  can be declared without being initialized	const  must be initialized during declaration

3. What are variables?

Variables are like containers for storing values. We can create variables using the keywords like

let

,

const

, etc.

4. What are global variables?

Global variables are the variables that are available throughout the length of the code without any scope.

5. What are the ways to define a variable in JavaScript?

The three ways to define a variable are:

• const: If a variable is declared using

  const
  , then initialization is mandatory. Once the variable is initialized with a value, then it can't be reassigned and it will have the block scope.

• let: If a variable is declared using

  let
  , then initialization is not mandatory. This variable can be reassigned and it will have the block scope.

• var: If a variable is declared using

  var
  , then initialization is not mandatory. This variable can be reassigned and redeclared. Before executing the code, all the variable declarations are processed at first.

6. What are the differences between var and let?

basis of comparison	var	let
Scope	Variables declared with  var  are scoped to the immediate function body (Function Scope)	variables declared with  let  are scoped to the immediate enclosing block denoted by  { }  (Block Scope)
Hoisting	Variables declared with  var  are hoisted	Variables declared with  let  are not hoisted. Accessing variables before the declaration results in a Reference Error
Redeclaration	Variables can be redeclared	Variables cannot be redeclared

7. What are the differences between let, const, var?

let	const	var
If a variable is declared using  let  then initialization is not mandatory	If a variable is declared using  const  then initialization is mandatory	If a variable is declared using  var  then initialization is not mandatory.
The variable can be reassigned	The variable can't be reassigned	The variable can be reassigned and redeclared
The variable will have the block scope	The variable will have the block scope	The variable will have global scope or function scope

8. Explain data types in js?

The set of data types in the JavaScript are primitive values and objects.

Examples for primitive data types:

• number
• string
• boolean
• null
• undefined
• symbol
• BigInt

Example for Object data type:

• object

9. Explain about undefined in JS?

Undefined

• If a value is not assigned to the variable, then it takes 
  undefined
   as its value.
• In JS, 
  undefined
   refers to the value that is not being assigned to a variable.

10. What are primitive data types in JS?

Primitive Types

• number
• string
• boolean
• null
• undefined
• symbol

11. What are the differences between null and undefined values?

null	undefined
null  can be assigned to a variable as a representation of no value. It is used when we intentionally want a variable but don't need a value to it	undefined  means a variable has been declared but has not yet been assigned to a value
The datatype of null is object	The datatype of undefined is undefined itself

12. What are the Datastructures in JavaScript?

Data Structures allow us to store and organize data efficiently. This makes us access and performs operations on the data smoothly.

In JavaScript, we have built-in Data Structures like,

• Arrays
• Objects
• Maps
• Sets

13. What are the differences between Primitive Type and Reference Type?

Primitive	Reference
Primitives include Number, String, Boolean, Symbol, Undefined, etc.	References include Objects. i.e arrays, objects, etc.
Primitives are immutable	References are mutable
Primitives are passed by value	References are passed by reference

14. What is the use of the string method Split?

The

split

method is used to split a string into an array of substrings and returns the new array.

Syntax:

string.split(separator)

Here, separator is a string used to separate substring of the string.

Example:

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const string = "How are you doing today?";

const result = string.split(" ");

console.log(result);

JAVASCRIPT

Output:

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["How", "are", "you", "doing", "today?"]

Quick Tip

If an

empty string ("")

is used as the separator, the string is split by each character.

15. What is the use of the array method Join?

The

join

method returns the array as a string. The elements will be joined by a specified separator.

Syntax:

arr.join(separator)

Here separator is a string used to separate each item of the array. If omitted, the array items are joined with a comma.

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const fruits = ["Banana", "Orange", "Apple", "Mango"];

const energy = fruits.join();

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Output:

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Banana,Orange,Apple,Mango

16. How to remove Whitespaces in a string?

We use the

trim

method to remove whitespace from both ends of a string.

Example:

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const greeting = " Hello world! ";

console.log(greeting);

console.log(greeting.trim()); // Output: Hello world!

JAVASCRIPT