Jest looping through dynamic test cases

Question

How do I loop through dynamic test cases in Jest?

I have test cases like the following how do I dynamically create jest test case using it/test methods.

Here is what I have tried , However it just passes without excuting the test cases in the loop.

    const mymodule = require('mymodule');

    const testCases = [
        {q: [2, 3],r: 5},
        {q: [1, 2],r: 3},
        {q: [7, 0],r: 7},
        {q: [4, 4],r: 8}
    ];

    describe("Test my Math module", () => {
        test("test add method", () => {
            for (let i = 0; i < testCases.length; i++) {
                const { q,r } = testCases[i];
                it(`should  add ${q[0]},${q[1]} to ${expected}`, () => {
                    const actual = mymodule.add(q[0] + q[1]);
                    expect(actual).toBe(expected);
                });
            }
        });
    
    });

Answer 1

There's an in-built way to do this: test.each(table)(name, fn, timeout)

e.g.

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

where each inner array in the 2D array is passed as args to the test function.

Answer 2

If one passes, they all will. So you only need one for a positive test. Ideally, you want 1 positive where the 2 numbers equal the sum, and a failure where you pass a string or something and throw an error.

TL/DR; You don't need 4 tests that all do the same thing. Also, you need to pull the looped test cases out of the parent test for the loop to work.

This will work just fine:

import jest from 'jest';
import { sumModule } from './';

const tests = [
  {x: 1, y: 2, r: 3}, 
  {x: 3, y: 4, r: 7}
];

describe('Adding method', () => { 
    for(let i = 0; i < tests.length; i++){
      it('should add its params', () => {
        const actual = sumModule(tests[i].x, tests[i].y);
        expect(actual).toBe(tests[i].r);
      });
    }
});

Answer 3

If anyone wondering how this can work for a single input function here is an example

const testData = [
      ['input1', 'output1'],
      ['input2', 'output2'],
      ['input3', 'output3'],
]

test.each(testData)('myFunc work correctly for %s',(input, output) =>{
        expect(yourFunc(input)).toBe(output)
})

https://jestjs.io/docs/en/api#testeachtablename-fn-timeout

Answer 4

You can use classic JS, it is more readable and with less code:

[
  { input: [2, 3], output: 5 },
  { input: [1, 2], output: 3 },
].forEach(({ input, output }) => {
  it(`works correctly for ${input}`, () => {
    // ...

    expect(...).toBe(output);
  });
})

Answer 5

An alternative solution:

const cases = [
  { q: [2, 3], r: 3 },
  { q: [1, 2], r: 3 },
  { q: [7, 0], r: 7 },
  { q: [4, 5], r: 8 }
];

describe("Test cases for math module", () => {
  cases.forEach((el, i) => {
    let value1 = el.q[0]
    let value2 = el.q[1]
    let value3 = el.r

    test(`the sum of ${value1} and ${value2} equals ${value3}`, () => {
      expect(value1 + value2).toBe(value3)
    })
  })
})

Test results

When all tests pass:

In case one or more tests fail, it will show like so:

I did all the tests in CodeSandbox, if you want to play with the code click here

Answer 6

Instead of going with native JavaScript forEach way, It's better to use test.each() provided by Jest framework.

Below are some basic examples

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

Please note that we can pass array of primitives, array of arrays or array of objects to test.each()

When we pass array of primitives like ['a', 'b', 'c'], it internally converts to [['a'], ['b'], ['c']]

Below is another example by using Template Literal

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(a + b).toBe(expected);
});

Answer 7

For loop works in jest, No need to use inbuilt function without any reason!

 test('adding positive numbers is not zero', () => {
  for (let a = 1; a < 10; a++) {
    for (let b = 1; b < 10; b++) {
      expect(a + b).not.toBe(0);
    }
  }
});