feat: mm.spy method (#51)

README.md

@@ -60,16 +60,38 @@ const target = {
 
 mm.data(target, 'add', 3);
 
-(await target.add(1, 1)).should.equal(3);
-target.add.called.should.equal(1);
-target.add.calledArguments.should.eql([[ 1, 1 ]]);
-target.add.lastCalledArguments.should.eql([ 1, 1 ]);
-
-(await target.add(2, 2)).should.equal(3);
-target.add.called.should.equal(2);
-target.add.calledArguments.should.eql([[ 1, 1 ], [ 2, 2 ]]);
-target.add.lastCalledArguments.should.eql([ 2, 2 ]);
-});
+assert.equal(await target.add(1, 1), 3);
+assert.equal(target.add.called, 1);
+assert.deepEqual(target.add.calledArguments, [[ 1, 1 ]]);
+assert.deepEqual(target.add.lastCalledArguments, [ 1, 1 ]);
+
+assert.equal(await target.add(2, 2), 3);
+assert.equal(target.add.called, 2);
+assert.deepEqual(target.add.calledArguments, [[ 1, 1 ], [ 2, 2 ]]);
+assert.deepEqual(target.add.lastCalledArguments, [ 2, 2 ]);
+```
+
+If you only need spy and don't need mock, you can use `mm.spy` method directly:
+
+```js
+const target = {
+  async add(a, b) {
+    await this.foo();
+    return a + b;
+  },
+  async foo() { /* */ },
+};
+
+mm.spy(target, 'add');
+assert.equal(await target.add(1, 1), 2);
+assert.equal(target.add.called, 1);
+assert.deepEqual(target.add.calledArguments, [[ 1, 1 ]]);
+assert.deepEqual(target.add.lastCalledArguments, [ 1, 1 ]);
+
+assert.equal(await target.add(2, 2), 4);
+assert.equal(target.add.called, 2);
+assert.deepEqual(target.add.calledArguments, [[ 1, 1 ], [ 2, 2 ]]);
+assert.deepEqual(target.add.lastCalledArguments, [ 2, 2 ]);
 ```
 
 ### Support generator function

lib/mm.js

@@ -217,6 +217,16 @@ exports.empty = function(mod, method, timeout) {
   return exports.datas(mod, method, null, timeout);
 };
 
+/**
+ * spy a function
+ * @param {Object} mod, module object
+ * @param {String} method, mock module object method name.
+ */
+exports.spy = function(mod, method) {
+  if (typeof mod[method] !== 'function') throw new Error(`spy target ${method} is not a function`);
+  mock(mod, method, mod[method]);
+};
+
 /**
  * mock function sync throw error
  *

test/mm.test.js

@@ -1086,7 +1086,25 @@ describe('test/mm.test.js', () => {
       target.add(2, 2).should.equal(3);
       assert(target.add === mockAdd);
     });
+
+    it('should mm.spy() work', async () => {
+      const target = {
+        async add(a, b) {
+          await this.foo();
+          return a + b;
+        },
+        async foo() { /* */ },
+      };
+
+      mm.spy(target, 'add');
+      (await target.add(1, 1)).should.equal(2);
+      (await target.add(2, 2)).should.equal(4);
+      target.add.called.should.equal(2);
+      target.add.calledArguments.should.eql([[ 1, 1 ], [ 2, 2 ]]);
+      target.add.lastCalledArguments.should.eql([ 2, 2 ]);
+    });
   });
+
 });
 
 const enable = require('enable');