How to perform parameterized testing in Jest? How to use test.each and describe.each?

Jest provides methods for parameterized testing using test.each or describe.each to run multiple test data sets:

1. Using test.each for Parameterized Testing:

javascript
test.each([
  [1, 1, 2],
  [1, 2, 3],
  [2, 1, 3],
])('adds %i + %i = %i', (a, b, expected) => {
  expect(add(a, b)).toBe(expected);
});

2. Using Object Arrays:

javascript
test.each([
  { a: 1, b: 1, expected: 2 },
  { a: 1, b: 2, expected: 3 },
  { a: 2, b: 1, expected: 3 },
])('$a + $b = $expected', ({ a, b, expected }) => {
  expect(add(a, b)).toBe(expected);
});

3. Using describe.each for Grouped Testing:

javascript
describe.each([
  ['node', 'node'],
  ['jsdom', 'browser'],
])('test environment: %s', (env, type) => {
  test(`runs in ${type} environment`, () => {
    expect(process.env.NODE_ENV).toBeDefined();
  });
});

4. Testing Table Data:

javascript
test.each`
  a    | b    | expected
  ${1} | ${1} | ${2}
  ${1} | ${2} | ${3}
  ${2} | ${1} | ${3}
`('returns $expected when $a is added to $b', ({ a, b, expected }) => {
  expect(add(a, b)).toBe(expected);
});

5. Testing Edge Cases:

javascript
test.each([
  [0, 0, 0],
  [Number.MAX_SAFE_INTEGER, 1, Number.MAX_SAFE_INTEGER + 1],
  [Number.MIN_SAFE_INTEGER, -1, Number.MIN_SAFE_INTEGER - 1],
])('handles edge cases: %i + %i = %i', (a, b, expected) => {
  expect(add(a, b)).toBe(expected);
});

6. Testing Error Cases:

javascript
test.each([
  [undefined, 'input is required'],
  [null, 'input is required'],
  ['', 'input cannot be empty'],
])('throws error for invalid input: %p', (input, expectedError) => {
  expect(() => validate(input)).toThrow(expectedError);
});

Best Practices:

• Use parameterized testing to reduce code duplication
• Clearly describe test data and expected results
• Test normal cases and edge cases
• Use table format for better readability
• Keep test data concise and clear