Interview question

What is NaN and how do you check for it correctly? NaN क्या है और इसे सही तरीके से कैसे check करें?

Answer

NaN stands for 'Not-a-Number' but is of type 'number'. It's the only value in JavaScript that is not equal to itself!

// What produces NaN
const a = 0 / 0;           // NaN
const b = parseInt('hello'); // NaN
const c = undefined + 5;    // NaN
const d = Math.sqrt(-1);    // NaN

// WRONG way to check for NaN
if (a == NaN) console.log('This will NEVER run!'); // Always false
if (a === NaN) console.log('This will NEVER run!'); // Always false
if (typeof a === 'number' && a != a) console.log('Hacky way'); // Works but ugly

// RIGHT way to check for NaN
if (isNaN(a)) console.log('Correct!'); // true
if (Number.isNaN(a)) console.log('Even better!'); // true - strict version

// Difference between isNaN and Number.isNaN
console.log(isNaN('hello'));         // true (converts to number first)
console.log(Number.isNaN('hello'));  // false (no type conversion)

Check method	Description	Type coercion?
NaN == NaN	WRONG - always false	N/A
isNaN()	Good, but coerces type	Yes
Number.isNaN()	BEST - strict, no coercion	No

Interview tip: NaN !== NaN is one of the most famous JavaScript quirks. Always use Number.isNaN() for reliable checking. Mention that typeof NaN === 'number' (weird!).

NaN का मतलब 'Not-a-Number' है पर यह 'number' type का है। यह JavaScript में एकमात्र value है जो अपने आप के बराबर नहीं है!

// NaN क्या produce करता है
const a = 0 / 0;           // NaN
const b = parseInt('hello'); // NaN
const c = undefined + 5;    // NaN

// गलत तरीका
if (a == NaN) console.log('यह कभी नहीं चलेगा'); // Always false
if (a === NaN) console.log('यह कभी नहीं चलेगा'); // Always false

// सही तरीका
if (isNaN(a)) console.log('सही है'); // true
if (Number.isNaN(a)) console.log('और भी बेहतर'); // true

// isNaN vs Number.isNaN में अंतर
console.log(isNaN('hello'));         // true (convert करता है)
console.log(Number.isNaN('hello'));  // false (convert नहीं करता)

Check method	Description	Type coercion?
NaN == NaN	गलत - हमेशा false	N/A
isNaN()	अच्छा, पर coerces	हाँ
Number.isNaN()	सबसे अच्छा - strict	नहीं

इंटरव्यू टिप: NaN !== NaN एक famous JavaScript quirk है। हमेशा Number.isNaN() use करें। typeof NaN === 'number' का ज़िक्र करें।

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