string

The value to parse, whitespace in this argument is ignored.

A floating point number parsed from the given string, or NaN when the first non-whitespace character cannot be converted to a number.

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.

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() {
    return "3.14";
  },
});

The following example returns NaN:

parseFloat("FF2");

Anecdotally, because the string NaN itself is invalid syntax as accepted by parseFloat(), passing "NaN" returns NaN as well.

parseFloat("NaN"); / NaN

Infinity values are returned when the number is outside the double-precision 64-bit IEEE 754-2019 format range:

parseFloat("1.7976931348623159e+308"); / Infinity
parseFloat("-1.7976931348623159e+308"); / -Infinity

Infinity is also returned when the string starts with "Infinity" or "-Infinity":

parseFloat("Infinity"); / Infinity
parseFloat("-Infinity"); / -Infinity

parseFloat() 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. If a BigInt value is passed to parseFloat(), it will be converted to a string, and the string will be parsed as a floating-point number, which may result in loss of precision as well.

parseFloat(900719925474099267n); / 900719925474099300
parseFloat("900719925474099267n"); / 900719925474099300

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

BigInt("900719925474099267");
/ 900719925474099267n

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