[javascript] How to listen state changes in react.js?

What is the angular's $watch function equivalent in React.js?

I want to listen state changes and call a function like getSearchResults().

componentDidMount: function() {
    this.getSearchResults();
}

The following lifecycle methods will be called when state changes. You can use the provided arguments and the current state to determine if something meaningful changed.

componentWillUpdate(object nextProps, object nextState)
componentDidUpdate(object prevProps, object prevState)

Using useState with useEffect as described above is absolutely correct way. But if getSearchResults function returns subscription then useEffect should return a function which will be responsible for unsubscribing the subscription . Returned function from useEffect will run before each change to dependency(name in above case) and on component destroy

I think you should be using below Component Lifecycle as if you have an input property which on update needs to trigger your component update then this is the best place to do it as its will be called before render you even can do update component state to be reflected on the view.

componentWillReceiveProps: function(nextProps) {
  this.setState({
    likesIncreasing: nextProps.likeCount > this.props.likeCount
  });
}

In 2020 you can listen state changes with useEffect hook like this

export function MyComponent(props) {
    const [myState, setMystate] = useState('initialState')

    useEffect(() => {
        console.log(myState, '- Has changed')
    },[myState]) // <-- here put the parameter to listen
}

It's been a while but for future reference: the method shouldComponentUpdate() can be used.

An update can be caused by changes to props or state. These methods are called in the following order when a component is being re-rendered:

static getDerivedStateFromProps() 
shouldComponentUpdate() 
render()
getSnapshotBeforeUpdate() 
componentDidUpdate()

ref: https://reactjs.org/docs/react-component.html

Since React 16.8 in 2019 with useState and useEffect Hooks, following are now equivalent (in simple cases):

AngularJS:

$scope.name = 'misko'
$scope.$watch('name', getSearchResults)

<input ng-model="name" />

React:

const [name, setName] = useState('misko')
useEffect(getSearchResults, [name])

<input value={name} onChange={e => setName(e.target.value)} />

If you use hooks like const [ name , setName ] = useState (' '), you can try the following:

 useEffect(() => {
      console.log('Listening: ', name);
    }, [name]);