Stubbing Math.random()

vector.js

export function random(vmin, vmax) {
  return [
    vmin[0] + Math.random() * (vmax[0] - vmin[0]),
    vmin[1] + Math.random() * (vmax[1] - vmin[1])
  ];
}

vector.test.js

import * as V from './vector';
import td from 'testdouble';

it('returns a random vector within the bounds given', () => {
  const rand = td.replace(Math, 'random');
  td.when(rand()).thenReturn(0.2, 0.8);
  const vmin = [-50, -100];
  const vmax = [50, 0];
  const actual = V.random(vmin, vmax);
  expect(actual).toEqual([-30, -20]);
  td.reset();
});