# isFinite function

The `isFinite` function checks if a parameter is a finite number (that is, not a string, array, etc., and not plus or minus infinity).

How this function works: it will return `false` if the number is plus or minus infinity (i.e. `Infinity`) or not a number (i.e. `NaN`), otherwise it will return `true`. That is, strings, arrays, etc. will be converted to `NaN` and will return `false` accordingly.

However, there are exceptions: an empty string `''` returns true, the string `' '` with spaces also returns true, `null` returns true, for the values `true` and `false` it also returns `true`.

This is because these values are converted to numbers and not to `NaN`. If you need a really accurate check for a number that does not count a string of spaces, booleans and special values as a number - use the following `isNumeric` function:

`function isNumeric(num) { return !isNaN(parseFloat(num)) && isFinite(num); };`

Let's see how it works. The `isFinite` function converts the parameter to a number and returns `true` if it is not `Infinity`, `-Infinity`, or `NaN`. Thus, the right side will definitely filter out non-numbers, but leave such values as `true`, `false`, `null`, an empty string `''` and a string with spaces, because they are correctly converted to numbers.

To filter out these values, you need the `parseFloat` function, which for `true`, `false`, `null`, `''`, `' '` will return `NaN`. This is how the `parseFloat` function works: it converts a parameter to a string, i.e. `true`, `false`, `null` become `'true'`, `'false'`, `'null'` and then reads a number from it, with an empty string and a string with spaces giving `NaN`. The result of `parseFloat` is then processed with !isNaN to get `true` or `false` instead of `NaN`. As a result, everything is filtered out, except for strings-numbers and ordinary numbers.

## Syntax

`isFinite(value);`

## Example

Now `isFinite` will output `true` since the parameter is a number:

`let num = 3; console.log(isFinite(num));`

The code execution result:

`true`

## Example

Now `isFinite` will output `false` since the parameter is not a number:

`let num = 'abcde'; console.log(isFinite(num));`

The code execution result:

`false`

## Example

Now `isFinite` will output `false` since the parameter is infinity:

`let num = Infinity; console.log(isFinite(num));`

The code execution result:

`false`

## Example

Now `isFinite` will output `false`, since `1/0` is essentially `Infinity`:

`let num = 1 / 0; console.log(isFinite(num));`

The code execution result:

`false`

## Example

Now `isFinite` will output `true` as an empty string that is not a number is an exception:

`let num = ''; console.log(isFinite(num));`

The code execution result:

`true`

## See also

- the `isNaN` function
  that checks for `NaN`
- the `typeof` operator
  that defines a data type

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