Fn

The xJet.fn() utilities let you create highly configurable mocks for functions, constructors, or methods. These mocks let you track calls, control behavior, and replace implementation logic for powerful and isolated testing scenarios.

Overview

• Quickly mock standalone functions, methods, or class constructors.
• Capture call arguments, return values, invocation order, and more.
• Swap between original and mocked behavior with .mockRestore().
• Support for both function-like and constructor-like mocks.

Basic Function Mock

test('some test', () => {
    const double = (x: number) => x * 2;
    const mockDouble = xJet.fn(double);
    mockDouble.mockReturnValueOnce(7);

    expect(mockDouble(10)).toBe(7);
    expect(mockDouble(10)).toBe(20);
});

Mocking a Class Constructor

test('some test', () => {
    class User {
        name: string;
        
        constructor(name: string) {
            this.name = name;
        }

        sayHello() {
            return `Hi, ${ this.name }`;
        }
    }

    const userMock = xJet.fn<User, [ string ], User>();
    userMock.mockImplementation((name: string) => ({ name, sayHello: () => `Mocked ${ name }` }));
    
    
    const mockInstance = new userMock('Alice');
    expect(mockInstance.sayHello()).toBe('Mocked Alice');
});

API Overview

A mock created via xJet.fn (or .mock) exposes all MockState methods and properties for fine-grained control:

Method / Property	Description
getMockName()	Returns the name of the mock (mainly for Jest compatibility).
mock	Read-only state with calls, instances, results, and more.
original	Returns the original, unmocked function or constructor.
mockClear()	Clears all recorded call data, keeping the mock implementation.
mockReset()	Resets the mock completely: clears calls, results, and queued behaviors.
mockRestore()	Restores the function/constructor to its original (pre-mocked) implementation.
getMockImplementation()	Gets the current implementation of the mock, or undefined if none is set.
getNextImplementation()	Gets the next implementation in the queue, or falls back to the default mock/current impl.
mockImplementation(fn)	Sets or replaces the mock's default function/logic.
mockImplementationOnce(fn)	Adds a one-time function/implementation for the next call only.
mockReturnValue(val)	Always returns the specified value.
mockReturnValueOnce(val)	Returns value for the next call only, then reverts.
mockResolvedValue(val)	Always returns a resolved Promise with value when called.
mockResolvedValueOnce(val)	Next call returns resolved Promise, then reverts.
mockRejectedValue(val)	Always returns a rejected Promise with value.
mockRejectedValueOnce(val)	Next call returns rejected Promise, then reverts.

Quick API Table

Task	How to Use
Create a mock function	const fn = xJet.fn() or xJet.fn(impl)
Mock a specific implementation	fn.mockImplementation(() => 5)
Mock a value	fn.mockReturnValue('foo')
Mock a resolved value	fn.mockResolvedValue('ok')
Mock a rejected value	fn.mockRejectedValue('err')
Restore original	fn.mockRestore()
Inspect calls	fn.mock.calls
Reset/clear collected state	fn.mockReset() / fn.mockClear()

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See Also

• Spy API
• Mock