parseFloat() - JavaScript 编辑

The parseFloat() function parses an argument (converting it to a string first if needed) and returns a floating point number.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

parseFloat(string)

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.

Return value

A floating point number parsed from the given string.

Or NaN when the first non-whitespace character cannot be converted to a number.

Description

parseFloat is a function property of the global object.

• If parseFloat encounters a character other than a plus sign (+), minus sign (- U+002D HYPHEN-MINUS), numeral (0–9), decimal point (.), or exponent (e or E), it returns the value up to that character, ignoring the invalid character and characters following it.
• A second decimal point also stops parsing (characters up to that point will still be parsed).
• Leading and trailing spaces in the argument are ignored.
• If the argument’s first character can’t be converted to a number (it’s not any of the above characters), parseFloat returns NaN.
• parseFloat can also parse and return Infinity.
• parseFloat converts BigInt syntax to Numbers, losing precision. This happens because the trailing n character is discarded.

Consider Number(value) for stricter parsing, which converts to NaN for arguments with invalid characters anywhere.

parseFloat will parse non-string objects if they have a toString or valueOf method. The returned value is the same as if parseFloat had been called on the result of those methods.

Examples

parseFloat returning a number

The following examples all return 3.14:

parseFloat(3.14);
parseFloat('3.14');
parseFloat('  3.14  ');
parseFloat('314e-2');
parseFloat('0.0314E+2');
parseFloat('3.14some non-digit characters');
parseFloat({ toString: function() { return "3.14" } });

parseFloat returning NaN

The following example returns NaN:

parseFloat('FF2');

parseFloat and BigInt

The following examples both return 900719925474099300, losing precision as the integer is too large to be represented as a float:

parseFloat(900719925474099267n);
parseFloat('900719925474099267n');

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'parseFloat' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• parseInt()
• Number.parseFloat()
• Number.parseInt()
• Number.toFixed()
• isNaN()