Infinity and NaN in JavaScript: Usage and Examples

In JavaScript, Infinity represents a value larger than any number, often the result of dividing by zero, while NaN means "Not a Number" and indicates an invalid or undefined numerical operation. Both are special numeric values with unique behaviors used to handle exceptional cases in calculations.

Syntax

Infinity is a global property representing an infinite numeric value. NaN is a global property representing an invalid number result.

You can use them directly or get them as results of operations:

• Infinity - a numeric value representing infinity.
• NaN - a special value indicating an invalid number.

javascript

console.log(Infinity);
console.log(NaN);

console.log(1 / 0);      // Infinity
console.log('a' * 3);    // NaN

Output

Infinity
NaN
Infinity
NaN

Example

This example shows how Infinity and NaN appear in calculations and how to check for them.

javascript

const positiveInfinity = 1 / 0;
const negativeInfinity = -1 / 0;
const notANumber = 'hello' * 5;

console.log('Positive Infinity:', positiveInfinity);
console.log('Negative Infinity:', negativeInfinity);
console.log('NaN:', notANumber);

// Checking values
console.log('Is NaN:', Number.isNaN(notANumber));
console.log('Is finite:', Number.isFinite(positiveInfinity));

Output

Positive Infinity: Infinity
Negative Infinity: -Infinity
NaN: NaN
Is NaN: true
Is finite: false

Common Pitfalls

Common mistakes include confusing NaN with other falsy values and using equality checks that don't work as expected.

• NaN === NaN is false, so use Number.isNaN() to check for NaN.
• Dividing by zero returns Infinity, which is a number, not an error.
• Using isNaN() (global) can give unexpected results because it converts values before checking.

javascript

console.log(NaN === NaN);           // false (wrong way)
console.log(Number.isNaN(NaN));     // true (correct way)

console.log(isNaN('hello'));        // true (global isNaN converts string)
console.log(Number.isNaN('hello')); // false (does not convert)

console.log(1 / 0);                 // Infinity (not an error)

Output

false
true
true
false
Infinity

Quick Reference

Concept	Description	Check Method
Infinity	Represents a value larger than any number	Use Number.isFinite() to check if not infinite
-Infinity	Represents negative infinity	Same as Infinity check
NaN	Result of invalid numeric operations	Use Number.isNaN() to check
Division by zero	Returns Infinity or -Infinity, not error	No error thrown
NaN equality	NaN === NaN is false	Use Number.isNaN()

Key Takeaways

Infinity represents an unbounded number, often from division by zero.

NaN means an invalid number result and must be checked with Number.isNaN().

Never use === to check for NaN because it always returns false.

Division by zero returns Infinity, not an error.

Use Number.isFinite() to test if a value is a normal number, not Infinity or NaN.