React Test. how to test function inside react component

Question

What is the proper way to test a method within a react component, in this case componentDidMount. I want to test the setTimeOut function inside the compoenent. Should I use stub? For example the code below:

 componentDidMount() {
        setTimeout(() => this.setState({ isOpen: true }), 1);
  }

How can I test the setTimeout being called?

I tried with the following and didn't work. What am I missing?

my imports:

import test from 'ava';
import React from 'react';
import { ad } from 'components/Ad/Ad';
import { shallow, mount } from 'enzyme';
import { stub } from 'sinon';
import { expect } from 'chai';
import  sinon from 'sinon';

let info;

test.beforeEach(() => {

  info = shallow(<ad {...props} />)
});

test('is active: true after mounting', done => {
  info.instance().componentDidMount()
  setTimeout(() => {
    info.state('active').should.be.true  <--line:42
    done()
  }, 0)
})

I get the following error: TypeError: Cannot read property 'be' of undefined null._onTimeout (test/Components/Ad.unit.js:42:5)

Answer 1

Here's an example that uses mocha, chai, and enzyme:

The component:

import React, {PropTypes as T} from 'react'
import classnames from 'classnames'

export default class FadeIn extends React.Component {
  constructor(...args) {
    super(...args)
    this.state = {active: false}
  }

  componentDidMount() {
    setTimeout(() => this.setState({active: true}), 0)
  }

  render() {
    const {active} = this.state
    return (
      <div className={classnames('fade-in', {active}, this.props.className)}>
        {this.props.children}
      </div>
    )
  }
}

FadeIn.propTypes = {
  className: T.string
}

FadeIn.displayName = 'FadeIn'

The test:

import React from 'react'
import {shallow} from 'enzyme'
import FadeIn from '../../src/components/FadeIn'

describe('FadeIn', () => {
  let component

  beforeEach(() => {
    component = shallow(<FadeIn/>)
  })

  it('is initially active: false', () => {
    component.state('active').should.be.false
    component.find('div.fade-in').prop('className').should.equal('fade-in')
  })

  it('is active: true after mounting', done => {
    component.instance().componentDidMount()
    setTimeout(() => {
      component.state('active').should.be.true
      component.find('div.fade-in').prop('className').should.equal('fade-in active')
      done()
    }, 0)
  })

})