# parseInt()

The `parseInt()` function parses a string argument and returns an integer of the specified radix (the base in mathematical numeral systems).

## Syntax

``````parseInt(string)
``````

### Parameters

`string`

The value to parse. If this argument is not a string, then it is converted to one using the `ToString` abstract operation. Leading whitespace in this argument is ignored.

`radix` Optional

An integer between `2` and `36` that represents the radix (the base in mathematical numeral systems) of the `string`. Be careful—this does not default to `10`! If the radix value is not of the `Number` type it will be coerced to a `Number`.

Warning: The description below explains in more detail what happens when `radix` is not provided.

### Return value

An integer parsed from the given `string`.

Or `NaN` when

• the `radix` modulo `2**32` is smaller than `2` or bigger than `36`, or
• the first non-whitespace character cannot be converted to a number.

## Description

The `parseInt` function converts its first argument to a string, parses that string, then returns an integer or `NaN`.

If not `NaN`, the return value will be the integer that is the first argument taken as a number in the specified `radix`. (For example, a `radix` of `10` converts from a decimal number, `8` converts from octal, `16` from hexadecimal, and so on.)

For radices above `10`, letters of the English alphabet indicate numerals greater than `9`. For example, for hexadecimal numbers (base `16`), `A` through `F` are used.

If `parseInt` encounters a character that is not a numeral in the specified `radix`, it ignores it and all succeeding characters and returns the integer value parsed up to that point. `parseInt` truncates numbers to integer values. Leading and trailing spaces are allowed.

Because some numbers use the `e` character in their string representation (e.g. `6.022E23` for 6.022 × 10^23), using `parseInt` to truncate numbers will produce unexpected results when used on very large or very small numbers. `parseInt` should not be used as a substitute for `Math.floor()`.

`parseInt` understands exactly two signs: `+` for positive, and `-` for negative (since ECMAScript 1). It is done as an initial step in the parsing after whitespace is removed. If no signs are found, the algorithm moves to the following step; otherwise, it removes the sign and runs the number-parsing on the rest of the string.

A value passed as the radix argument is coerced to a Number (if necessary), then if the value is 0, `NaN` or `Infinity` (undefined is coerced to `NaN`), JavaScript assumes the following:

1. If the input `string` begins with "`0x`" or "`0X`" (a zero, followed by lowercase or uppercase X), `radix` is assumed to be `16` and the rest of the string is parsed as a hexadecimal number.
2. If the input `string` begins with any other value, the radix is `10` (decimal).

Else if the radix value (coerced if necessary) is not in range [2, 36] (inclusive) `parseInt` returns `NaN`.

If the first character cannot be converted to a number with the radix in use, `parseInt` returns `NaN`.

For arithmetic purposes, the `NaN` value is not a number in any radix. You can call the `isNaN` function to determine if the result of `parseInt` is `NaN`. If `NaN` is passed on to arithmetic operations, the operation result will also be `NaN`.

To convert a number to its string literal in a particular radix, use `thatNumber.toString(radix)`.

Warning: `parseInt` converts a `BigInt` to a `Number` and loses precision in the process. This is because trailing non-numeric values, including "`n`", are discarded.

### Octal interpretations with no radix

Please note that following information doesn't apply to recent implementations as of 2021.

Although discouraged by ECMAScript 3, many ECMAScript 3 implementations had interpreted a numeric string beginning with a leading `0` as octal. The following might have had an octal result, or it might have had a decimal result.

``````parseInt('0e0')  // 0
parseInt('08')   // 0, because '8' is not an octal digit.
``````

The ECMAScript 5 specification of the function `parseInt` no longer allows implementations to treat Strings beginning with a `0` character as octal values. Many implementations have adopted this behavior as of 2021.

``````parseInt('0e0')  // 0
parseInt('08')   // 8
``````

### A stricter parse function

It is sometimes useful to have a stricter way to parse integers.

Regular expressions can help:

``````function filterInt(value) {
if (/^[-+]?(\d+|Infinity)\$/.test(value)) {
return Number(value)
} else {
return NaN
}
}

console.log(filterInt('421'))                // 421
console.log(filterInt('-421'))               // -421
console.log(filterInt('+421'))               // 421
console.log(filterInt('Infinity'))           // Infinity
console.log(filterInt('421e+0'))             // NaN
console.log(filterInt('421hop'))             // NaN
console.log(filterInt('hop1.61803398875'))   // NaN
console.log(filterInt('1.61803398875'))      // NaN
``````

## Examples

### Using parseInt

The following examples all return `15`:

``````parseInt('0xF', 16)
parseInt('F', 16)
parseInt('17', 8)
parseInt(021, 8)
parseInt('015', 10)    // but `parseInt('015', 8)` will return 13
parseInt(15.99, 10)
parseInt('15,123', 10)
parseInt('FXX123', 16)
parseInt('1111', 2)
parseInt('15 * 3', 10)
parseInt('15e2', 10)
parseInt('15px', 10)
parseInt('12', 13)
``````

The following examples all return `NaN`:

``````parseInt('Hello', 8)  // Not a number at all
parseInt('546', 2)    // Digits other than 0 or 1 are invalid for binary radix
``````

The following examples all return `-15`:

``````parseInt('-F', 16)
parseInt('-0F', 16)
parseInt('-0XF', 16)
parseInt(-15.1, 10)
parseInt('-17', 8)
parseInt('-15', 10)
parseInt('-1111', 2)
parseInt('-15e1', 10)
parseInt('-12', 13)
``````

The following examples all return `4`.

``````parseInt(4.7, 10)
parseInt(4.7 * 1e22, 10)        // Very large number becomes 4
parseInt(0.00000000000434, 10)  // Very small number becomes 4
``````

If the number is greater than 1e+21 (including) or less than 1e-7 (including), it will return `1`. (when using radix 10).

``````parseInt(0.0000001,10);
parseInt(0.000000123,10);
parseInt(1e-7,10);
parseInt(1000000000000000000000,10);
parseInt(123000000000000000000000,10);
parseInt(1e+21,10);
``````

The following example returns `224`:

``````parseInt('0e0', 16)
``````

`BigInt` values lose precision:

``````parseInt('900719925474099267n')
// 900719925474099300
``````

`parseInt` doesn't work with numeric separators:

``````parseInt('123_456')
// 123
``````

The radix is coerced to a `Number`:

``````const obj = {
valueOf() {return 8}
};
parseInt('11', obj); // 9

obj.valueOf = function() {return 1};
parseInt('11', obj); // NaN

obj.valueOf = function() {return Infinity};
parseInt('11', obj); // 11
``````

## Browser compatibility

BCD tables only load in the browser