Add ui-util formatting & util (#76)

* Add ui-util formatting & util

* Update README

packages/ui-util/src/formatElapsed.spec.ts

@@ -0,0 +1,45 @@
+// Copyright 2017-2019 @polkadot/ui-util authors & contributors
+// This software may be modified and distributed under the terms
+// of the Apache-2.0 license. See the LICENSE file for details.
+
+import BN from 'bn.js';
+import { Compact, UInt } from '@polkadot/types';
+
+import formatElapsed from './formatElapsed';
+
+describe('formatElapsed', () => {
+  const start = 12345678;
+  const now = new Date(12345678);
+
+  it('formats a Date', () => {
+    expect(
+      formatElapsed(now, new Date(start + 9700))
+    ).toEqual('9.7s');
+  });
+
+  it('formats a BN', () => {
+    expect(
+      formatElapsed(now, new BN(start + 42700))
+    ).toEqual('42s');
+  });
+
+  it('formats a Compact', () => {
+    const C = Compact.with(UInt);
+
+    expect(
+      formatElapsed(now, new C(start + (5.3 * 60000)))
+    ).toEqual('5m');
+  });
+
+  it('formats a number', () => {
+    expect(
+      formatElapsed(now, start + (42 * 60 * 60000))
+    ).toEqual('42h');
+  });
+
+  it('formats defaults', () => {
+    expect(
+      formatElapsed()
+    ).toEqual('0.0s');
+  });
+});