find significant digits. Useful for approximate equality tests where you care about orders of magnitude

sigdigit.js

/*
 * sigDigitFind
 *
 * Determine position of most significant digit
 *
 * Returns a positive number for significant digits left of the radix
 *   and a negative number for right of the radix.
 *
 * @param {String} inputFloat A String (optionally a Number) representing a
 *                   floating point number.
 *  
 */

function sigdigitFind(inputFloat) {
    // .search() is 0 indexed, of course. We want 1 indexed pos
    // of most significant digit

    // Drop negative sign and coerce to string 
    inputFloat = (Math.abs(parseFloat(inputFloat, 10))).toString();
    // If significant digit is before the decimal point
    // return a positive number
    if (parseInt(inputFloat, 10) !== 0) {
      return +(inputFloat.search(/[1-9]/) + 1);
    }
    // We're searching a string, so we want to chop off the decimal point
    inputFloat = inputFloat.substring(inputFloat.indexOf('.') + 1);
    // Return a negative number otherwise
    return -(inputFloat.search(/[1-9]/) + 1);
}