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)
parseInt(string, radix)
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
and36
that represents the radix (the base in mathematical numeral systems) of thestring
. Be careful—this does not default to10
! If the radix value is not of theNumber
type it will be coerced to aNumber
.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
modulo2**32
is smaller than2
or bigger than36
, 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:
-
If the input
string
begins with "0x
" or "0X
" (a zero, followed by lowercase or uppercase X),radix
is assumed to be16
and the rest of the string is parsed as a hexadecimal number. -
If the input
string
begins with any other value, the radix is10
(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)
.
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
Specifications
Specification |
---|
ECMAScript Language Specification # sec-parseint-string-radix |
Browser compatibility
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