parseInt()

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

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.

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.

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).

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

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

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

• parseFloat()
• Number.parseFloat()
• Number.parseInt()
• isNaN()
• Number.toString()
• Object.valueOf