index.js

export const closestToZero = input => {
  if (!(Array.isArray(input) && input.length)) {
    return 0;
  }

  return input.reduce((prev, value) => {
    const ratio = Math.abs(prev / value);

    if (ratio > 1 || (ratio === 1 && value > 0)) {
      return value;
    }

    return prev;
  }, Infinity);
};

index.test.js

import { closestToZero } from './index';

describe('closestToZero', () => {
  it('returns the value closest to 0', () => {
    expect(closestToZero([8, 5, 10])).toEqual(5);
    expect(closestToZero([8, -5, 10])).toEqual(-5);
  });
  it('favours positive values', () => {
    expect(closestToZero([8, 5, -5, 10])).toEqual(5);
    expect(closestToZero([8, -5, 5, 10])).toEqual(5);
  });
  it('returns 0 if input is not an array or is empty', () => {
    expect(closestToZero([])).toEqual(0);
    expect(closestToZero({})).toEqual(0);
    expect(closestToZero(null)).toEqual(0);
    expect(closestToZero()).toEqual(0);
  });
});