Function callback

packages/react/src/__tests__/ReactChildren-test.js:518–521

(kid, index)

  it('should invoke callback with the right context', () => {
    let lastContext;
    const callback = function (kid, index) {
      lastContext = this;
      return this;
    };

    // TODO: Use an object to test, after non-object fragments has fully landed.
    const scopeTester = 'scope tester';