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


parseInt(string, radix);


The value to parse. If string is not a string, then it is converted to one. Leading whitespace in the string is ignored.
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.


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 decimal integer representation of 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.

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

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).


Using parseInt

The following examples all return 15:

parseInt(" 0xF", 16);
parseInt(" F", 16);
parseInt("17", 8);
parseInt(021, 8);
parseInt("015", 10);
parseInt(15.99, 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 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:

filterInt = function (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

  Created by Mozilla Contributors, license: CC-BY-SA 2.5