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

OK, the big difference is start from where they are coming from, so constructor is the constructor of your class in JavaScript, on the other side, getInitialState is part of the lifecycle of React.

constructor is where your class get initialised...

Constructor

The constructor method is a special method for creating and initializing an object created with a class. There can only be one special method with the name "constructor" in a class. A SyntaxError will be thrown if the class contains more than one occurrence of a constructor method.

A constructor can use the super keyword to call the constructor of a parent class.

In the React v16 document, they didn't mentioned any preference, but you need to getInitialState if you using createReactClass()...

Setting the Initial State

In ES6 classes, you can define the initial state by assigning this.state in the constructor:

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {count: props.initialCount};
  }
  // ...
}

With createReactClass(), you have to provide a separate getInitialState method that returns the initial state:

var Counter = createReactClass({
  getInitialState: function() {
    return {count: this.props.initialCount};
  },
  // ...
});

Visit here for more information.

Also created the image below to show few lifecycles of React Compoenents: