The Book of JavaScript, 2nd Edition

parseFloat() [FF, IE 3]

Converts a string to a floating-point number as long as the first character is a number. If the first character is not a number, parseFloat() returns NaN (not a number). If the string is a number followed by letters, parseFloat() grabs the first set of numbers in the string.

Example:

var the_string = "3.14etc";
var the_numbers = parseFloat(the_string);

The variable the_numbers contains 3.14.