Jest

describe("<ComponentName />", () => {
  beforeAll(() => { ... })
  beforeEach(() => 
    it("should ...", () => {
      render(<Base {...Base.args}/>)
    })
  })

Mocking

For some modules like next/router a default mock is better.

Mock a function	`const createCandyMock = jest.fn().mockReturnValue(value) const createCandyMock = jest.fn().mockImplementation(() => value) const createCandyMock = jest.fn().mockResolvedValue(value)`
Mock a module	`import module from "importName" jest.mock("importName") module.function.function mock`
Mock function calls	`createCandyMock.mock.calls` each call is list of arguments

Mocking API calls

Install jest-fetch-mock.

fetchMock.mockOnce(JSON.stringify(data), { status: status code})
const [url, request] = fetchMock.mock.calls[index]
const method = request?.method
const body = JSON.parse(request?.body)

Assertions

A negation modifier not can be used.

Array contains a subset of elements	`expect(array).arrayContaining(subarray)`
Object contains subset of properties	`expect(object).objectContaining(subobject)`
Assert resolved promises	`return expect(promise).resolves.assertion`
Assert rejected promises	`return expect(promise).rejects.assertion`
Exact value	`expect(any).toEqual(value)`
A mock function has been called	`expect(mockFn).toHaveBeenCalled() expect(mockFn).toHaveBeenCalledTimes(number) expect(mockFn).toHaveBeenCalledWith(arg1, arg2, arg3)`
Assert array or string length	`expect(array).toHaveLength(number)`
Assert types	`expect(any).toBeDefined() expect(any).toBeFalsy() expect(any).toBeNull() expect(any).toBeTruthy() expect(any).toBeUndefined() expect(any).toBeNaN() expect(any).toBeInstanceof()`
Array contains an item	`expect(any).toContain(value)`
Throws an error	`expect(() => action).toThrow(error message)`

Execution Modifiers

Avoid debug modifiers skip, and only.

Todo, for planned tests	`it.todo("should ...")`
Run for dataset	`it.each([data])("should ...", (args) => ...)`
Run tests concurrently	`it.concurrent("should ...", () => ...)`