#Mock instance

MockInstance is the type of all mock/spy instances, providing a rich set of APIs for controlling and inspecting mocks.

#getMockName

• Type: () => string

Returns the mock name string set by .mockName().

const fn = rs.fn();
fn.mockName('myMock');
expect(fn.getMockName()).toBe('myMock');

#mockName

• Type: (name: string) => MockInstance

Sets the mock name for this mock instance, useful for debugging and output.

const fn = rs.fn();
fn.mockName('logger');

#mockClear

• Type: () => MockInstance

Clears all information about every call (calls, instances, contexts, results, etc.).

const fn = rs.fn();
fn(1);
fn.mockClear();
expect(fn.mock.calls.length).toBe(0);

#mockReset

• Type: () => MockInstance

Clears all call information and resets the implementation to the initial state.

const fn = rs.fn().mockImplementation(() => 1);
fn.mockReset();
// Implementation is reset

#mockRestore

• Type: () => void

Restores the original method of a spied object (only effective for spies).

const obj = { foo: () => 1 };
const spy = rs.spyOn(obj, 'foo');
spy.mockRestore();

#Symbol.dispose

• Type: () => void

Calls mockRestore() when the mock leaves a using scope.

const obj = { foo: () => 1 };

{
  using spy = rs.spyOn(obj, 'foo').mockImplementation(() => 2);
  expect(obj.foo()).toBe(2);
}

expect(obj.foo()).toBe(1);

#getMockImplementation

• Type: () => Function | undefined

Returns the current mock implementation function, if any.

const fn = rs.fn(() => 123);
const impl = fn.getMockImplementation();

#mockImplementation

• Type: (fn: Function) => MockInstance

Sets the implementation function for the mock.

const fn = rs.fn();
fn.mockImplementation((a, b) => a + b);

#mockImplementationOnce

• Type: (fn: Function) => MockInstance

Sets the implementation function for the next call only.

const fn = rs.fn();
fn.mockImplementationOnce(() => 1);
fn(); // returns 1
fn(); // returns undefined

#withImplementation

• Type: (fn: Function, callback: () => any) => void | Promise<void>

Temporarily replaces the mock implementation while the callback is executed, then restores the original implementation.

const fn = rs.fn(() => 1);
fn.withImplementation(
  () => 2,
  () => {
    expect(fn()).toBe(2);
  },
);
expect(fn()).toBe(1);

#mockReturnThis

• Type: () => this

Makes the mock return this when called.

const fn = rs.fn();
fn.mockReturnThis();
const obj = { fn };
expect(obj.fn()).toBe(obj);

#mockReturnValue

• Type: (value: any) => MockInstance

Makes the mock always return the specified value.

const fn = rs.fn();
fn.mockReturnValue(42);
expect(fn()).toBe(42);

#mockReturnValueOnce

• Type: (value: any) => MockInstance

Makes the mock return the specified value for the next call only.

const fn = rs.fn();
fn.mockReturnValueOnce(1);
expect(fn()).toBe(1);
expect(fn()).toBe(undefined);

#mockResolvedValue

• Type: (value: any) => MockInstance

Makes the mock return a Promise that resolves to the specified value.

const fn = rs.fn();
fn.mockResolvedValue(123);
await expect(fn()).resolves.toBe(123);

#mockResolvedValueOnce

• Type: (value: any) => MockInstance

Makes the mock return a Promise that resolves to the specified value for the next call only.

const fn = rs.fn();
fn.mockResolvedValueOnce(1);
await expect(fn()).resolves.toBe(1);
await expect(fn()).resolves.toBe(undefined);

#mockRejectedValue

• Type: (error: any) => MockInstance

Makes the mock return a Promise that rejects with the specified error.

const fn = rs.fn();
fn.mockRejectedValue(new Error('fail'));
await expect(fn()).rejects.toThrow('fail');

#mockRejectedValueOnce

• Type: (error: any) => MockInstance

Makes the mock return a Promise that rejects with the specified error for the next call only.

const fn = rs.fn();
fn.mockRejectedValueOnce(new Error('fail'));
await expect(fn()).rejects.toThrow('fail');
await expect(fn()).resolves.toBe(undefined);

#mock

The context of the mock, including call arguments, return values, instances, contexts, etc.

const fn = rs.fn((a, b) => a + b);
fn(1, 2);
expect(fn.mock.calls[0]).toEqual([1, 2]);

#mock.calls

• Type: Array<Parameters<T>>

An array containing the arguments for each call to the mock function.

const fn = rs.fn((a, b) => a + b);
fn(1, 2);
fn(3, 4);
console.log(fn.mock.calls); // [[1, 2], [3, 4]]

#mock.instances

• Type: Array<ReturnType<T>>

An array containing the instances that have been instantiated from the mock (when used as a constructor).

const Fn = rs.fn(function () {
  this.x = 1;
});
const a = new Fn();
const b = new Fn();
console.log(Fn.mock.instances); // [a, b]

#mock.contexts

• Type: Array<ReturnType<T>>

An array containing the this context for each call to the mock function.

const fn = rs.fn();
const context = {};

fn.apply(context);
fn.call(context);

fn.mock.contexts[0] === context;
fn.mock.contexts[1] === context;

#mock.invocationCallOrder

• Type: Array<number>

An array of numbers representing the order in which the mock was called, shared across all mocks. The index starts from 1.

const fn1 = rs.fn();
const fn2 = rs.fn();
fn1();
fn2();
fn1();
console.log(fn1.mock.invocationCallOrder); // [1, 3]
console.log(fn2.mock.invocationCallOrder); // [2]

#mock.lastCall

• Type: Parameters<T> | undefined

The arguments of the last call to the mock function, or undefined if it has not been called.

const fn = rs.fn();
fn(1, 2);
fn(3, 4);
console.log(fn.mock.lastCall); // [3, 4]

#mock.results

• Type: Array<MockResult<ReturnType<T>>>

An array containing the result of each call to the mock function, including returned values, thrown errors, or incomplete calls.

const fn = rs.fn((a, b) => a + b);
fn(1, 2);
try {
  fn();
  throw new Error('fail');
} catch {}
console.log(fn.mock.results);
// [{ type: 'return', value: 3 }, { type: 'throw', value: Error }]

#mock.settledResults

• Type: Array<MockSettledResult<Awaited<ReturnType<T>>>>

An array containing the settled results (fulfilled or rejected) of all async calls to the mock function.

const fn = rs.fn(async (x) => {
  if (x > 0) return x;
  throw new Error('fail');
});
await fn(1);
try {
  await fn(0);
} catch {}
console.log(fn.mock.settledResults);
// [{ type: 'fulfilled', value: 1 }, { type: 'rejected', value: Error }]