If the props for a child component are unchanged, does React still re-render it?

Question

Suppose I have the following pairing of parent and child components in React:

var ChildComponent = React.createClass({
    getDefaultProps: function(){
        return {
            a: 'a',
            b: 'b',
            c: 'c'
        }
    },

    render: function() {
        return (
            /* jshint ignore:start */
            <div className={'child' + (this.props.b ? ' modifierClass' : '')} something={this.props.a}>
                {this.props.c}
            </div>
            /* jshint ignore:end */
        );
    }
});


var ParentComponent = React.createClass({
    componentDidMount: function(){
        //After 10 seconds, change a property that DOES NOT affect the child component, and force an update
        setTimeout(function(){
            this.setState({foo: 'quux'});
            this.forceUpdate();
        }.bind(this), 10000);
    }

    getInitialState: function(){
        return {
            foo: 'bar',
            a: 1,
            b: 2,
            c: 3
        }
    },

    render: function() {
        return (
            /* jshint ignore:start */
            <div className="parent">
                <ChildComponent a={this.props.a} b={this.props.b} c={this.props.c}/>
            </div>
            /* jshint ignore:end */
        );
    }
});


React.render(
    /* jshint ignore:start */
    <ParentComponent />, 
    /* jshint ignore:end */
    document.getElementsByTagName('body')[0]
);

When I do the forceUpdate, since none of the props that were passed to the ChildComponent changed, will React try to re-render it? What if I have 1000 such children, etc?

What I'm worried about is a situation where I have a very deep ChildComponent containing a whole massive tree of descendant components, but I only want to enact some relatively cosmetic changes on the ParentComponent. Is there any way to get React to update only the parent, without trying to re-render the children as well?

Solution

When React re-renders ParentComponent it will automatically re-render ChildComponent. The only way to get around is to implement shouldComponentUpdate in the ChildComponent. You should compare this.props.a, this.props.b and this.props.c and ChildComponents own state to decide to re-render or not. If you are using immutable data than you can just compare previous and next state and props using strict equality ===.

There are a few thing to note with your code

1. You dont need to forceUpdate when you setState. React automatically does it for you.

2. You probably meant:

   <ChildComponent a={this.props.a} b={this.props.b} c={this.props.c}/>

Comments

[AlexZ]

Wow, that's exactly what I was looking for! Follow up question - is there any way to update the parent component while still allowing shouldComponentUpdate to execute on the children? Currently, when my Backbone model emits a change event, my ParentComponent executes a forceUpdate. Is there anything I can do to "force" an update, but still invoke shouldComponentUpdate on the children?

[AlexZ]

React docs beg to differ. When using forceUpdate, it says that shouldComponentUpdate is ignored.

[nilgun]

shouldComponentUpdate is called automatically on the ChildComponent, you dont have to do anything about it. forceUpdate skips the shouldComponentUpdate method of ParentComponent.

[nilgun]

here is a jsfiddle: jsfiddle.net/p7v59d0r. You can look at the logs on the console.

[AlexZ]

Ah, gotcha! Thanks for the fiddle too!