parseFloat()

The parseFloat() function parses a string argument and returns a floating point number.

Syntax

parseFloat(string)

Parameters

• string
  • : The value to parse, coerced to a string. 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.

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.

Description

The parseFloat function converts its first argument to a string, parses that string as a decimal number literal, then returns a number or NaN. The number syntax it accepts can be summarized as:

• The characters accepted by parseFloat() are plus sign (+), minus sign (- U+002D HYPHEN-MINUS), decimal digits (0 – 9), decimal point (.), exponent indicator (e or E), and the "Infinity" literal.
• The +/- signs can only appear strictly at the beginning of the string, or immediately following the e/E character. The decimal point can only appear once, and only before the e/E character. The e/E character can only appear once, and only if there is at least one digit before it.
• Leading spaces in the argument are trimmed and ignored.
• parseFloat() can also parse and return Infinity or -Infinity if the string starts with "Infinity" or "-Infinity" preceded by none or more white spaces.
• parseFloat() picks the longest substring starting from the beginning that generates a valid number literal. If it encounters an invalid character, it returns the number represented up to that point, ignoring the invalid character and all characters following it.
• If the argument's first character can't start a legal number literal per the syntax above, parseFloat returns NaN.

Syntax-wise, parseFloat() parses a subset of the syntax that the Number() function accepts. Namely, parseFloat() does not support non-decimal literals with 0x, 0b, or 0o prefixes but supports everything else. However, parseFloat() is more lenient than Number() because it ignores trailing invalid characters, which would cause Number() to return NaN.

Similar to number literals and Number(), the number returned from parseFloat() may not be exactly equal to the number represented by the string, due to floating point range and inaccuracy. For numbers outside the -1.7976931348623158e+308 – 1.7976931348623158e+308 range (see Number.MAX_VALUE), -Infinity or Infinity is returned.