Add reusable component for counting down

app/assets/javascripts/vue_shared/components/gl_countdown.vue

+<script>
+import { calculateRemainingMilliseconds, formatTime } from '~/lib/utils/datetime_utility';
+
+/**
+ * Counts down to a given end date.
+ */
+export default {
+  props: {
+    endDate: {
+      type: String,
+      required: true,
+    },
+  },
+
+  data() {
+    return {
+      remainingTime: formatTime(0),
+      countdownUpdateIntervalId: null,
+    };
+  },
+
+  mounted() {
+    const updateRemainingTime = () => {
+      const remainingMilliseconds = calculateRemainingMilliseconds(this.endDate);
+      this.remainingTime = formatTime(remainingMilliseconds);
+    };
+
+    updateRemainingTime();
+    this.countdownUpdateIntervalId = window.setInterval(updateRemainingTime, 1000);
+  },
+
+  beforeDestroy() {
+    window.clearInterval(this.countdownUpdateIntervalId);
+  },
+};
+</script>
+
+<template>
+  <time
+    v-gl-tooltip
+    :datetime="endDate"
+    :title="endDate"
+  >
+    {{ remainingTime }}
+  </time>
+</template>

spec/javascripts/vue_shared/components/gl_countdown_spec.js

+import mountComponent from 'spec/helpers/vue_mount_component_helper';
+import Vue from 'vue';
+import GlCountdown from '~/vue_shared/components/gl_countdown.vue';
+
+describe('GlCountdown', () => {
+  const Component = Vue.extend(GlCountdown);
+  let vm;
+  let now = '2000-01-01T00:00:00Z';
+
+  beforeEach(() => {
+    spyOn(Date, 'now').and.callFake(() => new Date(now).getTime());
+    jasmine.clock().install();
+  });
+
+  afterEach(() => {
+    vm.$destroy();
+    jasmine.clock().uninstall();
+  });
+
+  describe('when there is time remaining', () => {
+    beforeEach(done => {
+      vm = mountComponent(Component, {
+        endDate: '2000-01-01T01:02:03Z',
+      });
+
+      Vue.nextTick()
+        .then(done)
+        .catch(done.fail);
+    });
+
+    it('displays remaining time', () => {
+      expect(vm.$el).toContainText('01:02:03');
+    });
+
+    it('updates remaining time', done => {
+      now = '2000-01-01T00:00:01Z';
+      jasmine.clock().tick(1000);
+
+      Vue.nextTick()
+        .then(() => {
+          expect(vm.$el).toContainText('01:02:02');
+          done();
+        })
+        .catch(done.fail);
+    });
+  });
+
+  describe('when there is no time remaining', () => {
+    beforeEach(done => {
+      vm = mountComponent(Component, {
+        endDate: '1900-01-01T00:00:00Z',
+      });
+
+      Vue.nextTick()
+        .then(done)
+        .catch(done.fail);
+    });
+
+    it('displays 00:00:00', () => {
+      expect(vm.$el).toContainText('00:00:00');
+    });
+  });
+});