Jest Node.js and React

Here's a Jest cheat sheet summarizing its key features for writing effective unit tests in your Node.js and React projects:

1. Test Structure:

• it function: Defines a single test case.
  • describe block (optional): Groups related test cases.

describe("MyComponent", () => {
  it("should render correctly", () => {
    // Your test code here
  });
});

2. Matchers:

• Basic matchers:
  • toBe: Checks strict equality (===)
  • toEqual: Checks deep equality (regardless of object structure)
  • toBeNull
  • toBeUndefined
  • toBeTruthy
  • toBeFalsy
• Number matchers:
  • toBeGreaterThan
  • toBeLessThan
  • toBeCloseTo (allows for a precision parameter)
• String matchers:
  • toContain (checks if a substring exists)
  • toMatch (checks for a regular expression match)
• Array/Object matchers:
  • toContainEqual (checks if an array contains all elements in another array)
  • toMatchObject (checks if an object contains all properties and values in another object)

3. Mocking:

• jest.fn(): Creates a mock function for testing dependencies.
• Mock assertions:
  • toHaveBeenCalled: Checks if the function was called.
  • toHaveBeenCalledWith: Checks if the function was called with specific arguments.
  • toHaveReturnedWith: Checks the function's return value.

4. Snapshot Testing:

• toMatchSnapshot: Takes a snapshot of a value and compares it to a previously saved snapshot.
• Useful for ensuring UI components render consistently.

5. Async/Await Testing:

• Jest supports both async/await syntax and the callback approach with done for asynchronous testing.

Additional Resources:

• Official Jest Documentation: https://jestjs.io/docs/getting-started
• Interactive Jest Cheat Sheet: https://devhints.io/

Remember, this is a concise overview. Refer to the official documentation for in-depth information and examples for each feature.