What is the difference between using constructor vs getInitialState in React / React Native?

Question

I've seen both used interchangeably.

What are the main use cases for both? Are there advantages / disadvantages? Is one a better practice?

Answer 1

The two approaches are not interchangeable. You should initialize state in the constructor when using ES6 classes, and define the getInitialState method when using React.createClass.

See the official React doc on the subject of ES6 classes.

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = { /* initial state */ };
  }
}

is equivalent to

var MyComponent = React.createClass({
  getInitialState() {
    return { /* initial state */ };
  },
});

Note that in the constructor, you should always assign to this.state directly, and that's the only place where this is allowed; You should use this.setState(...) everywhere else.