doccano/frontend/plugins/color.ts

import Vue from 'vue'

declare module 'vue/types/vue' {
  interface Vue {
    $contrastColor(hexString: string): string
  }
}

Vue.prototype.$contrastColor = (hexString: string) => {
    // W3c offers a formula for calculating ideal color:
  // https://www.w3.org/TR/AERT/#color-contrast
  const r = parseInt(hexString.substr(1, 2), 16)
  const g = parseInt(hexString.substr(3, 2), 16)
  const b = parseInt(hexString.substr(5, 2), 16)
  return ((((r * 299) + (g * 587) + (b * 114)) / 1000) < 128) ? '#ffffff' : '#000000'
}