JS number formatting (thousands and uniform decimal places)

formatNumber.js

var formatNumber = function( value, decimalPlaces, alwaysDecimalize ) {
  // Set default decimal formatting values if undefined
  //     decimalPlaces defaults to 0
  //         Sets the number of places after the decimal point, if necessary
  //     alwaysDecimalize defaults to false
  //         If true, always shows a decimal and the set number of decimal
  //          places, even for integral values (useful with currency values)
  decimalPlaces = typeof decimalPlaces === 'undefined' ? 0 : decimalPlaces;
  alwaysDecimalize = (typeof alwaysDecimalize === 'undefined' ? false :
    alwaysDecimalize);

  if ( value === Number.POSITIVE_INFINITY ) {
    return '\u221e';
  } else if ( value === Number.NEGATIVE_INFINITY ) {
    return '-\u221e';
  } else if ( isNaN( value ) ) {
    return 'N/A';
  }

  var commasToAdd,
    decimalPart = '',
    firstComma,
    i,
    signPart = '',
    wholePart = Math.floor( Math.abs( value ) ) + '',  // Coerce to string.
    withCommas = wholePart,
    withCommasLength = withCommas.length;

  // Add a negative sign if the number is negative.
  if ( value < 0 ) {
    signPart = '-';
  }

  // Find the part after the decimal point.
  if ( alwaysDecimalize || value % 1 !== 0 ) {
    decimalPart = ( Math.abs( value ) % 1 ).toFixed( decimalPlaces );
    decimalPart = decimalPart.substring( 1 );  // Remove leading zero.
  }

  // Figure out how many commas we need to add.
  commasToAdd = Math.floor( withCommasLength / 3 );
  if ( withCommas.length % 3 === 0 ) {
    commasToAdd -= 1;
  }

  // Now that we know how many commas to add, add them!
  for ( i = 0; i < commasToAdd; i++ ) {
    firstComma = withCommas.indexOf( ',' );
    if ( firstComma >= 0 ) {
      withCommas = withCommas.substring( 0, firstComma - 3 ) +
        ',' + withCommas.substring( firstComma - 3);
    } else {
      withCommasLength = withCommas.length;
      withCommas = withCommas.substring( 0, withCommasLength - 3 ) +
        ',' + withCommas.substring( withCommasLength - 3 );
    }
  }
  return signPart + withCommas + decimalPart;
};