NaN and Infinity

Learn what NaN and Infinity mean and how to check for them.

NaN: Not a Number

NaN stands for Not a Number. It shows up when a number operation cannot produce a meaningful number.

Common causes:

parseInt("hello") — no number in the string.
0 / 0 — an indeterminate division.
Number("42px") — the whole string is not a valid number.

A quirk: NaN === NaN returns false! That is why you check NaN with the Number.isNaN method.

Infinity: Endless

Infinity represents numbers too large (or, when negative, -Infinity) to fit the limit. The most common source is division by zero: 1 / 0 gives Infinity.

Watch both ends: -1 / 0 gives -Infinity. These are still of type number; typeof Infinity returns "number".

To test whether a value is finite you use Number.isFinite(x). It returns false for NaN and Infinity, and true for normal numbers.

Exercise Check a division by zero

Declare const result = 10 / 0;. Then use Number.isFinite(result) to put the finiteness test into check. Log check with console.log.

Show example solution

const result = 10 / 0;
const check = Number.isFinite(result);
console.log(check);

Test yourself

Which one gives NaN?
- 5 / 1
- parseInt("hello")
- 1 / 0
- Number(42)
Show answer

parseInt("hello") — Correct. "hello" holds no number, so parseInt returns NaN.

Check NaN safely

const bad = Number.isNaN(Number(____));

Show answer

const bad = Number.isNaN(Number("abc"));

Match each value to its meaning

NaN

Not a number
Infinity

Endless value
Number.isNaN

Safe NaN check
1 / 0

Gives Infinity
What does Number.isNaN("hello") return?
```
console.log(Number.isNaN("hello"));
```
- true
- undefined
- Throws an error
- false
Show answer

false — Correct. Number.isNaN checks the type first; "hello" is a string, not NaN, so it returns false.

Next lesson: Converting to numbers

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