How does Increment and decrement work in JS?

JavaScript Operators

Introduction

Operators are special symbols in JavaScript that perform operations on values and variables.

For example:

let a = 10;
let b = 5;

console.log(a + b);

Output:

15

Here, + is an operator used to add two values.

JavaScript provides several types of operators that help us perform arithmetic calculations, comparisons, logical operations, assignments, and much more.

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1. Arithmetic Operators

Arithmetic operators are used to perform mathematical calculations.

Operator	Description	Example
+	Addition	10 + 5
-	Subtraction	10 - 5
*	Multiplication	10 * 5
/	Division	10 / 5
%	Modulus (Remainder)	10 % 3
**	Exponentiation	2 ** 3

Example

let a = 10;
let b = 5;

console.log(a + b);
console.log(a - b);
console.log(a * b);
console.log(a / b);
console.log(a % b);
console.log(a ** b);

Output:

15
5
50
2
0
100000

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Special Behavior of +

The + operator performs both addition and string concatenation.

console.log("Bahubali" + 2);

Output:

Bahubali2

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Type Coercion in Arithmetic Operations

console.log("15" - 5);

Output:

10

JavaScript automatically converts "15" into the number 15.

"15" → 15
15 - 5 = 10

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2. Assignment Operators

Used to assign values to variables.

Operator	Example	Equivalent
=	a = 10	Assign value
+=	a += 5	a = a + 5
-=	a -= 5	a = a - 5
*=	a *= 5	a = a * 5
/=	a /= 5	a = a / 5

Example

let a = 10;

a += 5;

console.log(a);

Output:

15

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3. Comparison Operators

Comparison operators compare two values and return either true or false.

Operator	Description
==	Equal to
===	Strict equal to
!=	Not equal
!==	Strict not equal
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal

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Double Equals (==)

Checks only values.

console.log(5 == "5");

Output:

true

Reason:

"5" → 5
5 == 5

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Triple Equals (===)

Checks both value and datatype.

console.log(5 === "5");

Output:

false

Reason:

Number !== String

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4. Logical Operators

Logical operators are commonly used in conditions.

Operator	Meaning
&&	AND
!	NOT

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AND (&&)

Returns true only if all conditions are true.

let age = 25;

console.log(age > 18 && age < 60);

Output:

true

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OR (||)

Returns true if at least one condition is true.

console.log(false || true);

Output:

true

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NOT (!)

Reverses the boolean value.

console.log(!true);

Output:

false

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5. Increment and Decrement Operators

Increment (++)

Adds 1.

let a = 5;
a++;

console.log(a);

Output:

6

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Decrement (--)

Subtracts 1.

let a = 5;
a--;

console.log(a);

Output:

4

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Pre-Increment vs Post-Increment

Post Increment

let a = 5;

console.log(a++);
console.log(a);

Output:

5
6

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Pre Increment

let a = 5;

console.log(++a);

Output:

6

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Interview Tricky Questions

Question 1

console.log("5" + 5);

Output:

55

Reason: String concatenation.

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Question 2

console.log("5" - 5);

Output:

0

Reason: String converted to number.

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Question 3

console.log(true + true);

Output:

2

Because:

true = 1
1 + 1 = 2

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Question 4

console.log(false + 10);

Output:

10

Because:

false = 0
0 + 10 = 10

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Question 5

console.log(10 + "5");

Output:

105

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Question 6

console.log(10 - "5");

Output:

5

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Question 7

console.log([] + []);

Output:

""

Empty string.

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Question 8

console.log([] + {});

Output:

"[object Object]"

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Question 9

console.log(null == undefined);

Output:

true

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Question 10

console.log(null === undefined);

Output:

false

Because strict equality checks datatype as well.

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Frequently Asked Interview Question

Difference Between == and ===

==	===
Checks value only	Checks value and datatype
Performs type conversion	No type conversion
Less strict	More strict
5 == "5" → true	5 === "5" → false

Interview Answer

== compares only values after type conversion, whereas === compares both values and datatypes without performing type conversion. Therefore, === is generally preferred in JavaScript applications.

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References :
https://www.geeksforgeeks.org/javascript/javascript-operators/
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_operators#arithmetic_operators