Mock functions or spies in Jest

MockFnFactory.js

// The mock function factory
function fn(impl = () => {}) {
  const mockFn = function(...args) {
    mockFn.mock.calls.push(args)
    mockFn.mock.instances.push(this)
    try {
      const value = impl.apply(this, args)
      mockFn.mock.results.push({ type: "return", value })
      return value
    }
    catch (value) {
      mockFn.mock.results.push({ type: "error", value })
      throw value
    }
  }
  mockFn.mock = {
    calls: [],
    instances: [],
    results: []
  }
  return mockFn
}