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isNaN vs Number.isNaN

Written byPhuoc Nguyen
26 Aug, 2023
JavaScript has two functions to determine if a value is `NaN` (standing for Not a Number): `isNaN` and `Number.isNaN`. Although they may seem similar at first glance, there are some important differences between them that are worth noting.

The isNaN function

The `isNaN` function in JavaScript is a handy global function that checks if the argument is `NaN`. It returns `true` if the argument is `NaN` and `false` if it's not. However, before checking if the argument is `NaN`, it first tries to convert it to a number. This can sometimes lead to unexpected results.
Let's explore how the `isNaN` function operates with various values.
isNaN('hello'); // true
isNaN(undefined); // true
isNaN({}); // true
isNaN([]); // false
isNaN(42); // false
As you see, the first three examples return `true` because `isNaN` tries to convert the argument to a number, but can't. In contrast, the last two examples return `false` because the argument can be converted to a number.

The Number.isNaN function

Introduced in ECMAScript 6, the `Number.isNaN` function is a static method of the `Number` object. It determines whether the provided argument is `NaN` and returns `true` if it is, and `false` if it is not. Unlike `isNaN`, which tries to convert the argument to a number before checking if it is `NaN`, `Number.isNaN` solely checks for `NaN` without any type coercion.
Let's go back to the example we saw earlier in the previous section, specifically with the `Number.isNaN` function.
Number.isNaN('hello'); // false
Number.isNaN(undefined); // false
Number.isNaN({}); // false
Number.isNaN([]); // false
Number.isNaN(42); // false
Number.isNaN(NaN); // true
Can you spot the difference between `Number.isNaN` and the global `isNaN` function? In all the examples above, `Number.isNaN` returns `false` except for the last one, where it returns `true`. Why? Because `NaN` is the only value that is not equal to itself. Pretty interesting, right?


It's generally better to use `Number.isNaN` than `isNaN` since it gives more accurate and predictable results. But, if you have to support older browsers that don't have `Number.isNaN`, then you might have to use `isNaN` instead.

See also

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Phước Nguyễn