Decimal to Hex in JavaScript

There's an easy way to get from decimal to hexadecimal in JavaScript:

  function toHex( n ) { return n.toString( 16 ); }

The string you get back may not look the way you want, though. For example, toHex(256) gives "100", when you're probably wanting "0x0100" or "0x00000100". What you need is front-padding. Just the right amount of front-padding.

// add just the right number of 'ch' characters
// to the front of string to give a new string of
// the desired final length 'dfl'

  function frontPad( string, ch, dfl ) {
     var array = new Array( ++dfl - string.length );
     return array.join( ch ) + string;
  }

Of course, you should ensure that 'dfl' is not smaller than string.length, to prevent a RangeError when allocating the array.

If you're wondering why "++dfl" instead of plain "dfl", stop now to meditate. Or run the code until enlightenment occurs.

At this point you can do:

  function toHex( n ) {
    return "0x" + frontPad( n.toString( 16 ), 0, 8);
  }

  toHex( 256 )  // gives "0x00000100" 

If you later need to use this value as a number, no problem. You can apply any numeric operation except addition on it with perfect safety. Addition will be treated as string concatenation whenever any operand is a string (that's the standard JS intepreter behavior), so if you need to do "0x00000100" + 4, you have to cast the hex-string to a number.

  n = toHex( 256 );  // "0x00000100"
  typeof n  // "string"
  isNaN( n )  // false
  x = n * n;  // 65536
  x = n + 256  // "0x00000100256"
  x = Number( n ) + 256   //  512