setup-test-framework-script.js

// Make sure chai and jasmine ".not" play nice together
const originalNot = Object.getOwnPropertyDescriptor(chai.Assertion.prototype, 'not').get;
Object.defineProperty(chai.Assertion.prototype, 'not', {
  get() {
    Object.assign(this, this.assignedNot);
    return originalNot.apply(this);
  },
  set(newNot) {
    this.assignedNot = newNot;
    return newNot;
  },
});

// Combine both jest and chai matchers on expect
const originalExpect = global.expect;

global.expect = (actual) => {
  const originalMatchers = originalExpect(actual);
  const chaiMatchers = chai.expect(actual);
  const combinedMatchers = Object.assign(chaiMatchers, originalMatchers);
  return combinedMatchers;
};

@JaKXz nice catch! Did you end up working something out with chai v4?

This appears to work alright for me now with jest 26.6.3 and chai 4.3.0. It even works after mixing in chai-as-promised, chai-string and chai-datetime!