modifyState.js

const item = 'FOOBerries!'

// Anti-pattern, don't do this!
this.state.list.push(item)
this.setState({ list: this.state.list })

// Correct way to do what the anti-pattern did:
const nextState = this.state.list.slice()
nextState.push(item)
this.setState({ list: nextState })

// ...but we can do better.

// Better! 
this.setState({
  list: [item, ...this.state.list]
})

// But sometimes we need to mutate more than just a simple array...

// Using immutability helper (https://github.com/kolodny/immutability-helper)
this.setState(
  update(this.state, {
    list: { $push: [item] }
  })
)