Add i18n unit tests

src/govuk/i18n.unit.test.mjs

@@ -0,0 +1,48 @@
+/**
+ * @jest-environment jsdom
+ */
+/* eslint-env jest */
+
+import I18n from './i18n.mjs'
+
+describe('I18n', () => {
+
+  describe('retrieving translations', () => {
+    let config = {}
+
+    beforeEach(() => {
+      config = {
+        textString: 'Hello world',
+        htmlString: 'Hello<span class="govuk-visually-hidden"> world</span>'
+      }
+    })
+
+    test('returns the text for a given lookup key', () => {
+      const i18n = new I18n(config)
+
+      let returnString = i18n.t('textString')
+      expect(returnString).toBe('Hello world')
+    })
+
+    test('returns the HTML for a given lookup key', () => {
+      const i18n = new I18n(config)
+
+      let returnString = i18n.t('htmlString')
+      expect(returnString).toBe('Hello<span class="govuk-visually-hidden"> world</span>')
+    })
+
+    test('returns the lookup key if no translation is defined', () => {
+      const i18n = new I18n(config)
+
+      let returnString = i18n.t('missingString')
+      expect(returnString).toBe('missingString')
+    })
+
+    test('throws an error if no lookup key is provided', () => {
+      const i18n = new I18n(config)
+
+      expect(() => i18n.t()).toThrow('i18n lookup key missing.')
+    })
+
+  })
+})