Jest notes

jest-notes.md

Jest

Mock

Mocking web APIs

Basic example

An example of how you could mock the fetch() function:

/// abc.test.js

window.fetch = jest
  .fn(() => (
    Promise.resolve({
      ok: true,
      json: () => {count: 1}
    })
  ));

By doing this, anytime any function makes a call to fetch(), Jest will run the mocked version above instead, returning a mock result object.

One step further

Taking this one step further:

/// abc.test.js

window.fetch = jest
  .fn()
  .mockImplementationOnce(() =>
    Promise.resolve({
      ok: true,
      json: () => {count: 1},
    })
  )
  .mockImplementationOnce(() =>
    Promise.resolve({
      ok: false,
      status: 404,
      statusText: "Not Found",
    })
  );

Doing this allows us to test for different fetch result cases (success and failure).