getRandomInt() & getRandomFloat()

#getRandomNumber.ts

// https://gist.github.com/tkrotoff/f3f36926edeeb3f4ce4411151bde37b2

// Exported for testing purposes only
// https://stackoverflow.com/a/45736131
export function getNumberWithDecimalPlaces(num: number, decimalPlaces: number) {
  const power = 10 ** decimalPlaces;
  return Math.floor(num * power) / power;
}

type GetRandomNumberOptions = {
  /**
   * The number of digits to appear after the decimal point.
   * https://ell.stackexchange.com/q/141863
   */
  decimalPlaces?: number;
};

/**
 * Generates a pseudo-random float (0.3546, 5.8557...) between min (included) and max (excluded).
 */
export function getRandomFloat(min: number, max: number, options: GetRandomNumberOptions = {}) {
  const { decimalPlaces } = options;

  const num = Math.random() * (max - min) + min;

  if (decimalPlaces === undefined) {
    return num;
  }

  return getNumberWithDecimalPlaces(num, decimalPlaces);
}

/**
 * Generates a pseudo-random integer (0, 6...) between min (included) and max (included).
 */
export function getRandomInt(min: number, max: number) {
  // https://stackoverflow.com/a/7228322
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

getRandomNumber.test.ts

import { getNumberWithDecimalPlaces, getRandomFloat, getRandomInt } from './getRandomNumber';

test('getRandomFloat()', () => {
  for (let i = 0; i < 100; i++) {
    expect(getRandomFloat(0, 1).toString()).toMatch(/^0\.\d+$/);

    expect([0, 5e-324]).toContain(getRandomFloat(0, 5e-324 /* Number.MIN_VALUE */));

    expect([0, 5e-324, 1e-323]).toContain(getRandomFloat(0, 1e-323));

    expect([0, 5e-324, 1e-323, 1.5e-323, 2e-323]).toContain(getRandomFloat(0, 2e-323));

    expect(getRandomFloat(0, 0)).toEqual(0);

    // Min/max inverted
    expect([0, 5e-324]).toContain(getRandomFloat(5e-324 /* Number.MIN_VALUE */, 0));
  }
});

test('getNumberWithDecimalPlaces()', () => {
  expect(getNumberWithDecimalPlaces(10.1234, 0)).toEqual(10);
  expect(getNumberWithDecimalPlaces(10.1234, 1)).toEqual(10.1);
  expect(getNumberWithDecimalPlaces(10.1234, 2)).toEqual(10.12);
  expect(getNumberWithDecimalPlaces(10.1234, 3)).toEqual(10.123);
  expect(getNumberWithDecimalPlaces(10.1234, 4)).toEqual(10.1234);
  expect(getNumberWithDecimalPlaces(10.1234, 5)).toEqual(10.1234);

  expect(getNumberWithDecimalPlaces(10.1234, -1)).toEqual(10);
  expect(getNumberWithDecimalPlaces(10.1234, -2)).toEqual(0);
  expect(getNumberWithDecimalPlaces(10.1234, -3)).toEqual(0);
});

test('getRandomFloat() with decimalPlaces', () => {
  for (let i = 0; i < 100; i++) {
    expect([0]).toContain(getRandomFloat(0, 1, { decimalPlaces: 0 }));

    expect([0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9]).toContain(
      getRandomFloat(0, 1, { decimalPlaces: 1 })
    );

    expect([0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09]).toContain(
      getRandomFloat(0, 0.1, { decimalPlaces: 2 })
    );
  }
});

test('getRandomInt()', () => {
  for (let i = 0; i < 100; i++) {
    expect([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]).toContain(getRandomInt(0, 10));
    expect([-10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0]).toContain(getRandomInt(-10, 0));

    expect([0, 1]).toContain(getRandomInt(0, 1));

    expect([0, 1]).toContain(getRandomInt(0, 0.1)); // Yep

    expect(getRandomInt(0, 0)).toEqual(0);

    // Min/max inverted
    expect([1, 2, 3, 4, 5, 6, 7, 8, 9]).toContain(getRandomInt(10, 0));
  }
});