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# 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, radix);`

### Parameters

`string`
The value to parse. If the `string` argument is not a string, then it is converted to a string (using the `ToString` abstract operation). Leading whitespace in the string argument is ignored.
`radix`
An integer between 2 and 36 that represents the radix (the base in mathematical numeral systems) of the above mentioned string. Specify `10` for the decimal numeral system commonly used by humans. Always specify this parameter to eliminate reader confusion and to guarantee predictable behavior. Different implementations produce different results when a radix is not specified, usually defaulting the value to 10.

### Return value

An integer number parsed from the given string. If the first character cannot be converted to a number, `NaN` is returned.

## Description

The `parseInt` function converts its first argument to a string, parses it, and returns an integer or `NaN`. If not `NaN`, the returned value will be the integer that is the first argument taken as a number in the specified radix (base). For example, a radix of 10 indicates to convert from a decimal number, 8 octal, 16 hexadecimal, and so on. For radices above `10`, the letters of the 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 include the `e` character in their string representation (e.g. `6.022e23`), using `parseInt` to truncate numeric values will produce unexpected results when used on very large or very small numbers. `parseInt` should not be used as a substitute for `Math.floor()`.

If radix is `undefined` or 0 (or absent), JavaScript assumes the following:

• If the input `string` begins with "0x" or "0X", radix is 16 (hexadecimal) and the remainder of the string is parsed.
• If the input `string` begins with "0", radix is eight (octal) or 10 (decimal).  Exactly which radix is chosen is implementation-dependent.  ECMAScript 5 specifies that 10 (decimal) is used, but not all browsers support this yet.  For this reason always specify a radix when using `parseInt`.
• If the input `string` begins with any other value, the radix is 10 (decimal).

If the first character cannot be converted to a number, `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 results will also be `NaN`.

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

## Examples

### Using `parseInt`

The following examples all return `15`:

```parseInt(' 0xF', 16);
parseInt(' F', 16);
parseInt('17', 8);
parseInt(021, 8);
parseInt('015', 10);   // parseInt(015, 10); will return 15
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 are not valid for binary representations
```

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
```

The following example returns `224`:

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

## Octal interpretations with no radix

Although discouraged by ECMAScript 3 and forbidden by ECMAScript 5, many implementations interpret a numeric string beginning with a leading `0` as octal. The following may have an octal result, or it may have a decimal result.  Always specify a radix to avoid this unreliable behavior.

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

### ECMAScript 5 removes octal interpretation

The ECMAScript 5 specification of the function `parseInt` no longer allows implementations to treat Strings beginning with a `0` character as octal values. ECMAScript 5 states:

The `parseInt` function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix is undefined or `0`, it is assumed to be `10` except when the number begins with the character pairs `0x` or `0X`, in which case a radix of 16 is assumed.

This differs from ECMAScript 3, which discouraged but allowed octal interpretation.

Many implementations have not adopted this behavior as of 2013, and because older browsers must be supported, always specify a radix.

## A stricter parse function

It is sometimes useful to have a stricter way to parse int values. Regular expressions can help:

```var filterInt = function(value) {
if (/^(\-|\+)?([0-9]+|Infinity)\$/.test(value))
return Number(value);
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
```

## Specifications

Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition.
ECMAScript 5.1 (ECMA-262)
The definition of 'parseInt' in that specification.
Standard
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'parseInt' in that specification.
Standard
ECMAScript Latest Draft (ECMA-262)
The definition of 'parseInt' in that specification.
Draft

## Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) ? (Yes)
Parses leading-zero strings as decimal, not octal (Yes) ? 21 (Yes) (in standards mode) ? (Yes)
Feature Android Chrome for Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Parses leading-zero strings as decimal, not octal ? ? ? 21 ? ? ?