A function for deeply setting values

set.js

function set(obj, path, value) {
  const clone = Object.assign({}, obj);
  const keys = path.split('.');
  const lastKey = keys.pop();

  // --------------------------------------------------

  if (keys.length === 0) {
    clone[lastKey] = value;
    return clone;
  }

  // --------------------------------------------------

  const firstKey = keys.shift();

  if (clone[firstKey] === undefined) {
    clone[firstKey] = {};
  }

  const secondToLastKeyRef = keys.reduce((keyRef, key) => {
    if (keyRef[key] === undefined) {
      keyRef[key] = {};
    }
    return keyRef[key];
  }, clone[firstKey]);

  secondToLastKeyRef[lastKey] = value;

  return clone;
}

// TESTS
describe('set(obj, path, value)', () => {
  it('returns an array with the path set to value', () => {
    expect(set({}, 'test', 'some string')).toEqual({ test: 'some string' });
  });

  it('works with objects', () => {
    expect(set({}, 'test', { hey: 1 })).toEqual({ test: { hey: 1 } });
  });

  it('works when the object is non-empty', () => {
    const obj = { hello: 'world' };
    expect(set(obj, 'test', false)).toEqual({
      hello: 'world',
      test: false,
    });
  });

  it('works with deeply nested values', () => {
    const obj = { lvl1: { lvl2: { lvl3: {} } } };
    expect(set(obj, 'lvl1.lvl2.lvl3.lvl4', 'hi')).toEqual(
      { lvl1: { lvl2: { lvl3: { lvl4: 'hi' } } } }
    );
  });

  it('creates deeply nested objects when needed', () => {
    expect(set({}, 'lvl1.lvl2.lvl3.lvl4', 'hi')).toEqual(
      { lvl1: { lvl2: { lvl3: { lvl4: 'hi' } } } }
    );
  });

  it('does not mutate the object passed in', () => {
    const obj = { hello: 'world' };
    set(obj, 'test', false);
    expect(obj).toEqual({ hello: 'world' });
  });
});