Add color utils with relevant tests

app/assets/javascripts/lib/utils/color_utils.js

@@ -0,0 +1,25 @@
+/**
+ * Convert hex color to rgb array
+ *
+ * @param hex string
+ * @returns array|null
+ */
+export const hexToRgb = hex => {
+  // Expand shorthand form (e.g. "03F") to full form (e.g. "0033FF")
+  const shorthandRegex = /^#?([a-f\d])([a-f\d])([a-f\d])$/i;
+  const fullHex = hex.replace(shorthandRegex, (_m, r, g, b) => r + r + g + g + b + b);
+
+  const result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(fullHex);
+  return result
+    ? [parseInt(result[1], 16), parseInt(result[2], 16), parseInt(result[3], 16)]
+    : null;
+};
+
+export const textColorForBackground = backgroundColor => {
+  const [r, g, b] = hexToRgb(backgroundColor);
+
+  if (r + g + b > 500) {
+    return '#333333';
+  }
+  return '#FFFFFF';
+};

spec/frontend/lib/utils/color_utils_spec.js

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+import { textColorForBackground, hexToRgb } from '~/lib/utils/color_utils';
+
+describe('Color utils', () => {
+  describe('Converting hex code to rgb', () => {
+    it('convert hex code to rgb', () => {
+      expect(hexToRgb('#000000')).toEqual([0, 0, 0]);
+      expect(hexToRgb('#ffffff')).toEqual([255, 255, 255]);
+    });
+
+    it('convert short hex code to rgb', () => {
+      expect(hexToRgb('#000')).toEqual([0, 0, 0]);
+      expect(hexToRgb('#fff')).toEqual([255, 255, 255]);
+    });
+
+    it('handle conversion regardless of the characters case', () => {
+      expect(hexToRgb('#f0F')).toEqual([255, 0, 255]);
+    });
+  });
+
+  describe('Getting text color for given background', () => {
+    // following tests are being ported from `text_color_for_bg` section in labels_helper_spec.rb
+    it('uses light text on dark backgrounds', () => {
+      expect(textColorForBackground('#222E2E')).toEqual('#FFFFFF');
+    });
+
+    it('uses dark text on light backgrounds', () => {
+      expect(textColorForBackground('#EEEEEE')).toEqual('#333333');
+    });
+
+    it('supports RGB triplets', () => {
+      expect(textColorForBackground('#FFF')).toEqual('#333333');
+      expect(textColorForBackground('#000')).toEqual('#FFFFFF');
+    });
+  });
+});