Create a new expect function that maintains a close relationship to the instance it is created from. The child instance has access to all assertions, types, and styles defined in the parent, even ones that are added after child() is called.

This is mostly useful when creating plugins that have secondary dependencies that can aid in implementing the plugin itself, but which should not leak out into the "primary" instance belonging to the user. This can avoid a class of dependency hell-like problems -- for instance when a plugin depends on another plugin that's also installed directly by the user at the top level, and most likely in an incompatible version.

The child instance has the ability to export assertions, types, and styles to the parent using the exportAssertion, exportType, and exportStyle methods. These methods have the same signature as addAssertion, addType, and addStyle, respectively, but affect the parent expect:

const childExpect = expect.child();
// Only available in childExpect:
  '<string> to begin with foo',
  function (expect, subject) {
    expect(subject, 'to begin with', 'foo');
// Available in parentExpect, but has access to "to begin with foo" internally:
childExpect.exportAssertion('<string> to foobar', function (expect, subject) {
  expect.errorMode = 'nested';
  expect(subject, 'to begin with foo').and('to end with', 'bar');
expect('fo0bar', 'to foobar');
expected 'fo0bar' to foobar
expected 'fo0bar' to begin with foo

Note that in order to reference a type in the signature used with exportAssertion, the type itself must also be exported to the parent expect. In other words, this won't work:

const childExpect = expect.child();
  name: 'foosomething',
  identify: function (value) {
    return /^foo/.test(String(value));
  '<foosomething> to end with bar',
  function (expect, subject) {
    expect(subject, 'to end with', 'bar');
Unknown type: foosomething in <foosomething> to end with bar

It can be fixed by exporting the foosomething type instead of just adding it to the child.