what is the preferred way to mutate a React state?

concat returns a new array, so you can do

this.setState({list: this.state.list.concat([newObject])});

another alternative is React's immutability helper

  var newState = React.addons.update(this.state, {
      list : {
        $push : [newObject]
      }
  });

  this.setState(newState);

setState() can be called with a function as a parameter:

this.setState((state) => ({ list: state.list.concat(newObj) }))

or in ES5:

this.setState(function(state) {
  return {
   list: state.list.concat(newObj)
  }
})

Update 2016

With ES6 you can use:

this.setState({ list: [...this.state.list, ...newObject] });

From the react docs (https://facebook.github.io/react/docs/state-and-lifecycle.html#state-updates-may-be-asynchronous):

Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.

So you should do this instead:

this.setState((prevState) => ({
  contacts: prevState.contacts.concat([contact])
}));

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