component.jsx

// @flow

import React from 'react'
import { fetchUsers, type User } from '../api/users'

type Props = { fetchUsers: typeof fetchUsers }

type State = { users: ?Array<User>}

class SmartComponent extends React.Component<Props, State> {
  static defaultProps = { fetchUsers }

  constructor(props: Props) {
    super(props)
    
    this.state = { users: null }
  }
  
  componentDidMount() {
    this.fetchUsers()
  }

  async fetchUsers() {
    const users = await this.props.fetchUsers()
    this.setState({ users })
  }

  render() {
    const { users } = this.props
    
    return users ? <div>{users.length}</div> : null
  }
}

export default SmartComponent

test_component.jsx

describe('<SmartComponent />', () => {
  it('should be possible to easily mock API calls', () => {
    const fetchUsers = jest.fn(() => Promise.resolve([]))
    const component = shallow(<SmartComponent fetchUsers={fetchUsers} />)
  })
})

💯