JavaScript NaN (Not-a-Number)

NaN means Not-a-Number. It is a special value of JavaScript's Number type used when a numeric operation cannot produce a meaningful numeric result.

const result = "xyz" * 2;
console.log(result); // NaN

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How NaN is produced Top ↑

Invalid arithmetic and failed numeric conversions are common sources of NaN.

console.log("hello" * 3);       // NaN
console.log(Number("hello"));      // NaN
console.log(parseFloat("hello"));  // NaN

Why typeof NaN is "number" Top ↑

NaN belongs to JavaScript's Number type even though it represents an invalid numeric result.

console.log(typeof NaN); // "number"

NaN is not equal to itself Top ↑

Both strict and loose equality comparisons with NaN are false, including comparison with another NaN.

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

Object.is() is one exception: it treats two NaN values as the same.

console.log(Object.is(NaN, NaN)); // true

How to check for NaN Top ↑

For most code, use Number.isNaN() when you need to know whether a value is exactly NaN.

const value = Number("abc");
if (Number.isNaN(value)) {
  console.log("Conversion failed");
}

The global isNaN() function performs numeric coercion first, so it answers a slightly different question.

How NaN propagates through calculations Top ↑

Once NaN enters most arithmetic operations, later results also become NaN. Finding the first failed conversion is therefore important when debugging calculations.

const price = NaN;
console.log(price + 10); // NaN
console.log(price * 2);  // NaN

NaN from parseInt() and parseFloat() Top ↑

console.log(parseInt("abc", 10)); // NaN
console.log(parseFloat("abc"));   // NaN

See parseInt() and parseFloat() for their partial-string parsing rules.

NaN in Boolean conditions Top ↑

NaN is falsy, but using truthiness alone is not a reliable numeric-validation strategy because 0 is also falsy.

console.log(Boolean(NaN)); // false
console.log(Boolean(0));   // false

Related tutorials Top ↑

Use Number.isNaN() and isNaN() to test values and continue with parseInt() when converting integer text.

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Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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