JavaScript illustration

In JavaScript, this expression might surprise you:

console.log(NaN === NaN); // false

Let’s break down why.

🔍 What is NaN?

NaN means “Not-a-Number”. It’s a special value used to represent the result of an invalid or undefined mathematical operation.

Examples:

console.log(0 / 0);         // NaN
console.log(Math.sqrt(-1)); // NaN

âť— Why is NaN !== NaN?

This behavior follows the IEEE 754 floating-point standard. According to this standard:

NaN is not equal to anything, not even itself.

This is intentional — it signals that the value is invalid and shouldn’t be trusted in comparisons.

So:

NaN === NaN; // false

Because both are “not-a-number,” JavaScript won’t consider them equal.

âś… How Do You Check for NaN?

Since regular equality checks won’t work, use these methods:

  1. isNaN() Checks if a value is NaN or can be coerced into NaN:

    isNaN("abc"); // true

  2. Number.isNaN() A safer way that only returns true if the value is strictly NaN:

    Number.isNaN(NaN); // true

🧠 In Summary

  • NaN === NaN returns false by design.
  • Use isNaN() or Number.isNaN() to safely check for NaN.
  • This behavior helps prevent silent math errors.

Review Questions

1. Why does NaN === NaN evaluate to false in JavaScript?

A) Because NaN is not a number B) Because JavaScript treats NaN as equal to itself C) Because the IEEE 754 standard specifies that NaN is not equal to any value, including itself D) Because NaN is undefined

2. Which of the following is the correct way to check if a value is NaN in JavaScript?

A) value == NaN B) value === NaN C) Number.isNaN(value) D) typeof value === 'NaN'

3. What is the type of NaN in JavaScript?

A) 'NaN' B) 'undefined' C) 'number' D) 'object'

4. Which of the following operations will result in NaN?

A) Math.sqrt(4) B) 0 / 0 C) parseInt('123') D) 5 + 5

5. Why is it important to use Number.isNaN() instead of isNaN() when checking for NaN?

A) Because isNaN() is not available in all browsers B) Because Number.isNaN() converts the argument to a number before checking C) Because isNaN() can return true for non-NaN values due to type coercion D) Because Number.isNaN() is faster than isNaN()

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