get the color (black or white) with the best contrast for the given color.

getContrastColor.js

export const getContrastColor = function (bgColor) {
  var color = (bgColor.charAt(0) === '#') ? bgColor.substring(1, 7) : bgColor
  var r = parseInt(color.substring(0, 2), 16) // hexToR
  var g = parseInt(color.substring(2, 4), 16) // hexToG
  var b = parseInt(color.substring(4, 6), 16) // hexToB
  
  var uiColors = [r / 255, g / 255, b / 255]
  var c = uiColors.map((col) => {
    if (col <= 0.03928) { return col / 12.92 }
    return Math.pow((col + 0.055) / 1.055, 2.4)
  })
  var L = (0.2126 * c[0]) + (0.7152 * c[1]) + (0.0722 * c[2])
  
  return (L > 0.3) ? '#000000' : '#FFFFFF'
}

demo: https://codesandbox.io/s/mfhtp

tuned slightly to match chrome's theme color contrast selection.