Add expect.js

expectjs.md

@@ -0,0 +1,29 @@
+---
+title: expect.js
+---
+
+```js
+expect(x).toBe(y)
+  .toBe(val)
+  .toEqual(val)
+  .toThrow(err)
+  .toExist
+  .toBeA(constructor)
+  .toBeA('string')
+  .toMatch(/expr/)
+  .toBeLessThan(n)
+  .toBeGreaterThan(n)
+  .toInclude(val)
+  .toExclude(val)
+```
+
+```js
+spy = expect.spyOn(video, 'play')
+expect(spy.calls.length).toEqual(1);
+expect(spy.calls[0].context).toBe(video);
+expect(spy.calls[0].arguments).toEqual([ 'some', 'args' ]);
+expect(spy).toHaveBeenCalled();
+expect(spy).toHaveBeenCalledWith('some', 'args');
+```
+
+https://www.npmjs.com/package/expect