ctx.mockImplementationOnce(implementation[, onCall])

• implementation <Function> | <AsyncFunction> 要用作 onCall 指定的调用编号的模拟实现的函数。
• onCall <integer> 将使用 implementation 的调用编号。 如果指定的调用已经发生，则抛出异常。 默认值: 下次调用的次数。

此函数用于更改单个调用的现有模拟的行为。 一旦调用 onCall 发生，模拟将恢复到没有调用 mockImplementationOnce() 时它会使用的任何行为。

以下示例使用 t.mock.fn() 创建模拟函数，调用模拟函数，将模拟实现更改为下一次调用的不同函数，然后恢复其先前的行为。

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);
});

• implementation <Function> | <AsyncFunction> The function to be used as the mock's implementation for the invocation number specified by onCall.
• onCall <integer> The invocation number that will use implementation. If the specified invocation has already occurred then an exception is thrown. Default: The number of the next invocation.

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);
});