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

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parseInt(string, radix)



A string starting with an integer. 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. It is converted to a 32-bit integer; if it's nonzero and outside the range of [2, 36] after conversion, the function will always return NaN. If 0 or not provided, the radix will be inferred based on string's value. Be careful — this does not always default to 10! 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 as a 32-bit integer is smaller than 2 or bigger than 36, or
  • the first non-whitespace character cannot be converted to a number.

Note: JavaScript does not have the distinction of "floating point numbers" and "integers" on the language level. parseInt() and parseFloat() only differ in their parsing behavior, but not necessarily their return values. For example, parseInt("42") and parseFloat("42") would return the same value: a Number 42.


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.)

The radix argument is converted to a number. If it's unprovided, or if the value becomes 0, NaN or Infinity (undefined is coerced to NaN), JavaScript assumes the following:

  1. If the input string, with leading whitespace and possible +/- signs removed, 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).

Note: Other prefixes like 0b, which are valid in number literals, are treated as normal digits by parseInt(). parseInt() does not treat strings beginning with a 0 character as octal values either. The only prefix that parseInt() recognizes is 0x or 0X for hexadecimal values — everything else is parsed as a decimal value if radix is missing. Number() or BigInt() can be used instead to parse these prefixes.

If the radix is 16, parseInt() allows the string to be optionally prefixed by 0x or 0X after the optional sign character (+/-).

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

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. The letters are case-insensitive.

parseInt understands exactly two signs: + for positive, and - for negative. 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.

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. For example, although 1e3 technically encodes an integer (and will be correctly parsed to the integer 1000 by parseFloat()), parseInt("1e3", 10) returns 1, because e is not a valid numeral in base 10. Because . is not a numeral either, the return value will always be an integer.

If the first character cannot be converted to a number with the radix in use, parseInt returns NaN. Leading whitespace is allowed.

For arithmetic purposes, the NaN value is not a number in any radix. You can call the Number.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.

Because large numbers use the e character in their string representation (e.g. 6.022e23 for 6.022 × 1023), 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.trunc().

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

Because parseInt() returns a number, it may suffer from loss of precision if the integer represented by the string is outside the safe range. The BigInt() function supports parsing integers of arbitrary length accurately, by returning a BigInt.


Using parseInt()

The following examples all return 15:

parseInt("0xF", 16);
parseInt("F", 16);
parseInt("17", 8);
parseInt("015", 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("-17", 8);
parseInt("-15", 10);
parseInt("-1111", 2);
parseInt("-15e1", 10);
parseInt("-12", 13);

The following example returns 224:

parseInt("0e0", 16);

parseInt() does not handle BigInt values. It stops at the n character, and treats the preceding string as a normal integer, with possible loss of precision.

// 900719925474099300

You should pass the string to the BigInt() function instead, without the trailing n character.

// 900719925474099267n

parseInt doesn't work with numeric separators:

parseInt("123_456"); // 123

Using parseInt() on non-strings

parseInt() can have interesting results when working on non-strings combined with a high radix; for example, 36 (which makes all alphanumeric characters valid numeric digits).

parseInt(null, 36); // 1112745: The string "null" is 1112745 in base 36
parseInt(undefined, 36); // 86464843759093: The string "undefined" is 86464843759093 in base 36

In general, it's a bad idea to use parseInt() on non-strings, especially to use it as a substitution for Math.trunc(). It may work on small numbers:

parseInt(15.99, 10); // 15
parseInt(-15.1, 10); // -15

However, it only happens to work because the string representation of these numbers uses basic fractional notation ("15.99", "-15.1"), where parseInt() stops at the decimal point. Numbers greater than or equal to 1e+21 or less than or equal to 1e-7 use exponential notation ("1.5e+22", "1.51e-8") in their string representation, and parseInt() will stop at the e character or decimal point, which always comes after the first digit. This means for large and small numbers, parseInt() will return a one-digit integer:

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

parseInt(0.0000001, 10); // 1
parseInt(0.000000123, 10); // 1
parseInt(1e-7, 10); // 1
parseInt(1000000000000000000000, 10); // 1
parseInt(123000000000000000000000, 10); // 1
parseInt(1e21, 10); // 1


ECMAScript Language Specification
# sec-parseint-string-radix

Browser compatibility

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See also