utils.js

export function padZero(str, len) {
  len = len || 2;
  var zeros = new Array(len).join('0');
  return (zeros + str).slice(-len);
}

export function invertColor(hex, bw) {
  if (hex.indexOf('#') === 0) {
    hex = hex.slice(1);
  }
  // convert 3-digit hex to 6-digits.
  if (hex.length === 3) {
    hex = hex[0] + hex[0] + hex[1] + hex[1] + hex[2] + hex[2];
  }
  if (hex.length !== 6) {
    throw new Error('Invalid HEX color.');
  }
  var r = parseInt(hex.slice(0, 2), 16),
    g = parseInt(hex.slice(2, 4), 16),
    b = parseInt(hex.slice(4, 6), 16);
  if (bw) {
    // https://stackoverflow.com/a/3943023/112731
    return (r * 0.299 + g * 0.587 + b * 0.114) > 186
      ? '#000000'
      : '#FFFFFF';
  }
  // invert color components
  r = (255 - r).toString(16);
  g = (255 - g).toString(16);
  b = (255 - b).toString(16);
  // pad each with zeros and return
  return "#" + padZero(r) + padZero(g) + padZero(b);
}