Reactjs – Why getDerivedStateFromProps is called after setState

getderivedstatefrompropsreactjs

React introduced new static method getDerivedStateFromProps(props, state) which is called before every render method, but why? Calling it after prop change makes sense to me but after setState it doesn't, maybe I am missing something.

I was creating a datePicker Component according to my company requirement, in the component date is controlled from the prop. I have the following state in the component.

selectedDate: number;
selectedMonth: number;
selectedYear: number;
currentMonth: number;
currentYear: number;
view: string;

selected represents selected date which is derived from date prop and currentMonth and currentYear represents month and year in the current calendar view.

If date from prop changes selected*, currentMonth and currentYear should be changed accordingly. For that, I am using getDerivedStateFromProps but let say user clicks on Month name which will switch calendar view to month (instead of dates name of the month will be shown), the function updates the currentMonth for this using setState, but date the prop is same as before (containing previous month) which should, but getDerivedStateFromProps is called and currentMonth is again as same as before instead of changing.

Right I creating an extra variable in state to track if getDerivedStateFromProps is called due to setState but I don't think that's the right way.

Either I am doing something wrong or missing something or getDerivedStateFromProps should not be called after setState. Probably I am doing something wrong.

Best Answer

The way getDerivedStateFromProps hook works whenever the new props, setState, and forceUpdate is being received.

In the version of 16.3, React is not affecting getDerivedStateFromProps whenever the setState is being used. But they improved it in the version starting with 16.4, so whenever the setState is being called the getDerivedStateFromProps is being hooked.

Here's extracted image from React lifecycle diagram:

16.3 ^16.4

So, it's up to you when to hook the getDerivedStateFromProps by checking props and states properly. Here's an example:

static getDerivedStateFromProps (props, state) {
  // check your condition when it should run?
  if(props.currentMonth != state.currentMonth) {
    return {
      currentMonth: state.currentMonth
    }
  }
  // otherwise, don't do anything
  else {
   return null
  }
}

Related Solutions

Javascript – ReactJS – Does render get called any time “setState” is called

Does React re-render all components and sub-components every time setState is called?

By default - yes.

There is a method boolean shouldComponentUpdate(object nextProps, object nextState), each component has this method and it's responsible to determine "should component update (run render function)?" every time you change state or pass new props from parent component.

You can write your own implementation of shouldComponentUpdate method for your component, but default implementation always returns true - meaning always re-run render function.

Quote from official docs http://facebook.github.io/react/docs/component-specs.html#updating-shouldcomponentupdate

By default, shouldComponentUpdate always returns true to prevent subtle bugs when the state is mutated in place, but if you are careful to always treat the state as immutable and to read-only from props and state in render() then you can override shouldComponentUpdate with an implementation that compares the old props and state to their replacements.

Next part of your question:

If so, why? I thought the idea was that React only rendered as little as needed - when the state changed.

There are two steps of what we may call "render":

Virtual DOM renders: when render method is called it returns a new virtual dom structure of the component. As I mentioned before, this render method is called always when you call setState(), because shouldComponentUpdate always returns true by default. So, by default, there is no optimization here in React.
Native DOM renders: React changes real DOM nodes in your browser only if they were changed in the Virtual DOM and as little as needed - this is that great React's feature which optimizes real DOM mutation and makes React fast.

Javascript – How to add textbox dynamically

var input = $('#input').clone().attr('name', 'name2').attr('id', 'input-2').appendTo('body')

You can go further and clone the entire row/div with $(el).clone() and then do .find('input') and modify the name and id attribute values so they're unique and don't conflict. You can pass true to clone if you want to copy the event handlers.

Incomplete non-jQuery "solution" since I don't know exactly at which point the OP is in since he claims he can clone nodes now..

  <div id="wrap">
    <div class="foo">
      <label for="first_name">name:</label><input type="text" name="first_name[]  " id="first_name"><a href="#">delete</a>
    </div>
    <a href="#" id="add">add name</a>
  </div>
  <script>
  (function() {
    var add = document.getElementById('add'), counter = 0;
    add.onclick = function() {
      var rows = document.getElementsByTagName('div'), last = false;
      if ( rows.length ) {
        for ( var i = rows.length; i--; ) {
            if ( last ) { break; }
            if ( rows[i].className.length && ( ' ' + rows[i].className + ' ' ).indexOf(' foo ') != -1 ) {
              last = rows[i];
            }
        }
      }
      if ( last ) {
        var newNode = last.cloneNode(true), wrap = document.getElementById('wrap'), input = newNode.getElementsByTagName('input');
        input.id = input.id + (counter++);
        wrap.appendChild( newNode );
      }
    }
  })();

Best Answer

16.3

^16.4

Related Solutions

Javascript – ReactJS – Does render get called any time “setState” is called

Javascript – How to add textbox dynamically

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