MockFunctionContext

The MockFunctionContext class is used to inspect or manipulate the behavior of mocks created via the MockTracker APIs.

A getter that returns a copy of the internal array used to track calls to the mock. Each entry in the array is an object with the following properties.

ctx.callCount(): integer

This function returns the number of times that this mock has been invoked. This function is more efficient than checking ctx.calls.length because ctx.calls is a getter that creates a copy of the internal call tracking array.

ctx.mockImplementation(implementation): void

This function is used to change the behavior of an existing mock.

The following example creates a mock function using t.mock.fn(), calls the mock function, and then changes the mock implementation to a different function.

test('changes a mock behavior', (t) => {
  let cnt = 0;

  function addOne() {
    cnt++;
    return cnt;
  }

  function addTwo() {
    cnt += 2;
    return cnt;
  }

  const fn = t.mock.fn(addOne);

  assert.strictEqual(fn(), 1);
  fn.mock.mockImplementation(addTwo);
  assert.strictEqual(fn(), 3);
  assert.strictEqual(fn(), 5);
});

ctx.mockImplementationOnce(implementation, onCall?): void

This function is used to change the behavior of an existing mock for a single invocation. Once invocation onCall has occurred, the mock will revert to whatever behavior it would have used had mockImplementationOnce() not been called.

The following example creates a mock function using t.mock.fn(), calls the mock function, changes the mock implementation to a different function for the next invocation, and then resumes its previous behavior.

test('changes a mock behavior once', (t) => {
  let cnt = 0;

  function addOne() {
    cnt++;
    return cnt;
  }

  function addTwo() {
    cnt += 2;
    return cnt;
  }

  const fn = t.mock.fn(addOne);

  assert.strictEqual(fn(), 1);
  fn.mock.mockImplementationOnce(addTwo);
  assert.strictEqual(fn(), 3);
  assert.strictEqual(fn(), 4);
});

ctx.resetCalls(): void

Resets the call history of the mock function.

ctx.restore(): void

Resets the implementation of the mock function to its original behavior. The mock can still be used after calling this function.