CSS string lightness

example.js

'#ddd'.darker // '#a0a0a0'
'#ddd'.lighter // '#f0f0f0'
'#ddd'.darker.darker // '#808080'

lightness.js

// please never actually do this

const lightness = ( str, mul ) => {
  if ( !/^#(([0-9a-f]{6})|([0-9a-f]{3}))$/i.test( str ) ) {
    return str;
  }

  const hex  = str.substr( 1 );
  const nums = hex.match( hex.length === 3 ? /.{1}/g : /.{1,2}/g );

  return nums.reduce( ( p, c ) => {
    const v = ~~( parseInt( c, 16 ) * mul );
    return p + Math.max( 0, Math.min( 255, v ) ).toString( 16 ).padEnd( 2, '0' );
  }, '#' );
};

Object.defineProperty( String.prototype, 'lighter', {
  get() {
    return lightness( String( this ), 1.2 );
  }
});

Object.defineProperty( String.prototype, 'darker', {
  get() {
    return lightness( String( this ), 0.8 );
  }
});