🧩 Understanding `isNaN()` vs `Number.isNaN()` in JavaScript – Complete Guide

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Short Description: Learn the key differences between isNaN() and Number.isNaN() in JavaScript. Understand type coercion, pitfalls, and best practices for handling NaN.

Keywords: JavaScript isNaN(), Number.isNaN(), type coercion, NaN handling, JavaScript numbers, ES6

🚀 Introduction

In JavaScript, handling NaN (Not-a-Number) values is a common source of confusion.

We have two different methods for checking NaN:

isNaN() – the older, global function.
Number.isNaN() – introduced in ES6 to fix isNaN() quirks.

Let’s break them down with examples and comparisons 👇

📌 What is `isNaN()`?

The isNaN() function checks whether a value is NaN. But before doing so, it tries to convert the value into a number.

✅ Syntax:

isNaN(value)

✅ Behavior:

true → if the converted value is NaN.
false → if the converted value is a valid number.

👉 Example:

isNaN("2");   // false → "2" → 2
isNaN("abc"); // true  → cannot convert → NaN

📌 What is `Number.isNaN()`?

The Number.isNaN() method was introduced in ES6. Unlike isNaN(), it does not coerce values.

✅ Syntax:

Number.isNaN(value)

✅ Behavior:

Returns true only if the value is literally NaN.
No type conversion happens.

👉 Example:

Number.isNaN("2");   // false → it's a string
Number.isNaN("abc"); // false → still a string
Number.isNaN(NaN);   // true  → exactly NaN

⚖️ Comparison: `isNaN()` vs `Number.isNaN()`

Value	`isNaN(value)`	`Number.isNaN(value)`	Why?
`NaN`	✅ true	✅ true	Both detect actual NaN
`"123"`	❌ false	❌ false	String → number works
`"ABC"`	✅ true	❌ false	Global `isNaN` coerces, `Number.isNaN` does not
`undefined`	✅ true	❌ false	`undefined → NaN` via coercion
`null`	❌ false	❌ false	`null → 0` when coerced
`{}`	✅ true	❌ false	Object → NaN in coercion
`[]`	❌ false	❌ false	Empty array → 0
`true`	❌ false	❌ false	`true → 1`

📝 Examples Side by Side

// Classic isNaN()
isNaN("Hello");      // true  → coerced into NaN
isNaN(undefined);    // true  → coerced into NaN

// Modern Number.isNaN()
Number.isNaN("Hello");   // false → it's a string
Number.isNaN(undefined); // false → it's undefined
Number.isNaN(NaN);       // true  → only works for NaN

🎯 Key Takeaways

isNaN() → coerces values before checking → can give unexpected results.
Number.isNaN() → strict check → only returns true for real NaN.
✅ Best Practice: Always prefer Number.isNaN() in modern JavaScript for reliability.

✍️ Written by Yogesh Bamanier
🔗 Connect with me on LinkedIn