jest-mock-random

Math.random() as deterministic Jest mock function.

Install

~ npm install --save-dev jest-mock-random
// or
~ yarn add --dev jest-mock-random

Usage

jest-mock-random can be used as an additional beforeEach for your sequence of test.

import { mockRandomForEach } from 'jest-mock-random';

describe('Test with random usage', () => {
  mockRandomForEach([0.1, 0.2, 0.3, 0.6]);
  it('assigns random the values that we want to mock in order', () => {
    const actual = [Math.random(), Math.random(), Math.random(), Math.random()]; // [0.1, 0.2, 0.3, 0.6]

    expect(actual).toEqual([0.1, 0.2, 0.3, 0.6]);
  });
});

mockRandomWith accept a single value that it will give back every time Math.random is called or an array of values that the mock will use as a circular array.

The values could be or a decimal number or a string that represents a decimal.

In the same way the mock can be use for individual test:

import { mockRandom, resetMockRandom } from 'jest-mock-random';

describe('Test with random usage', () => {
  it('assigns random the values that we want to mock in order', () => {
    mockRandom([0.1, 0.2]);
    const actual = [Math.random(), Math.random(), Math.random(), Math.random()]; // [0.1, 0.2, 0.1, 0.2]

    expect(actual).toEqual([0.1, 0.2, 0.1, 0.2]);

    resetMockRandom();
  });
});

WARNING: if a no decimal value is passed it will print some warnings during the test.

  mockRandomForEach([0.1, 0.2]); // OK
  mockRandomForEach([0.1, '0.2']); // OK
  mockRandomForEach(['0.2', '0.2']); // OK
  mockRandomForEach(0.2); // OK
  mockRandomForEach('0.2'); // OK

  mockRandomForEach('a'); // WARNING
  mockRandomForEach(13); // WARNING
  mockRandomForEach({}); // WARNING
  mockRandomForEach([]); // WARNING
  mockRandomForEach(['a', 1]); // WARNING