I've been playing around with React 16.6.0 recently and I love the idea of React Memo, but I've been unable to find anything regarding scenarios best suited to implement it.
The React docs (https://reactjs.org/docs/react-api.html#reactmemo) don't seem to suggest any implications from just throwing it on all of your functional components.
Because it does a shallow comparison to figure out if it needs to re-render, is there ever going to be a situation that negatively impacts performance?
A situation like this seems like an obvious choice for implementation:
// NameComponent.js
import React from "react";
const NameComponent = ({ name }) => <div>{name}</div>;
export default React.memo(NameComponent);
// CountComponent.js
import React from "react";
const CountComponent = ({ count }) => <div>{count}</div>;
export default CountComponent;
// App.js
import React from "react";
import NameComponent from "./NameComponent";
import CountComponent from "./CountComponent";
class App extends Component {
state = {
name: "Keith",
count: 0
};
handleClick = e => {
this.setState({ count: this.state.count + 1 });
};
render() {
return (
<div>
<NameComponent name={this.state.name} />
<CountComponent count={this.state.count} />
<button onClick={this.handleClick}>Add Count</button>
</div>
);
}
}
Because name will never change in this context, it makes sense to memoize.
But what about a situation where props change frequently?
What if I added another button that changed something else in state and triggered a re-render, would it make sense to wrap CountComponent in memo, even though this component by design is meant to update frequently?
I guess my main question is as long as everything remains pure, is there ever a situation to not wrap a functional component with React Memo?
React.memo LITERALLY, as comparing the tree returned by the Component is always more expensive than comparing a pair of props propertiesSo don't listen to anyone and wrap ALL functional components in React.memo. React.memo was originally intended to be built into the core of functional components, but it is not used by default due to the loss of backward compatibility. (Since it compares the object superficially, and you MAYBE are using the nested properties of the sub-object in the component) =)
That's it, this is the ONLY REASON why React doesn't use memo Automatically. =)
In fact, they could make version 17.0.0, which would BREAK backward compatibility, and make React.memo the default, and make some kind of function to cancel this behavior, for example React.deepProps =)
Stop listening to theorists, guys =) The rule is simple:
If your component uses DEEP COMPARING PROPS then don't use memo, otherwise ALWAYS use it, comparing TWO OBJECTS is ALWAYS CHEAPER than calling React.createElement() and comparing two trees, creating FiberNodes, and so on.
Theorists talk about what they themselves do not know, they have not analyzed the react code, they do not understand FRP and they do not understand what they're advising =)
P.S. if your component is using children prop, React.memo will not work, because children prop always makes a new array. But It is better not to bother about this, and even such components should ALSO be wrapped in React.memo, since the computing resources are negligible.
All react components implement the shouldComponentUpdate() method. By default (components extending React.Component), this returns true, always. The change that memoizing a component (through React.memo for functional components or extending React.PureComponent for class components) introduces is an implementation of the shouldComponentUpdate() method - which does the shallow comparison of the state and props.
Looking at the documentation on the component lifecycle methods, shouldComponentUpdate() is always called before the render happens, this means that memoizing a component will include this additional shallow comparison on every update.
Taking this into consideration, memoizing a component does have performance effects, and the magnitude of these effects should be determined through profiling your application and determining whether it works better with or without the memoizing.
To answer your question, I don't think there's an explicit rule when you should or should not memoize components, however I think the same principle should be applied as when deciding whether or not you should override shouldComponentUpdate(): find performance issues through the suggested profiling tools and identify whether or not you need to optimise a component.
Is there ever going to be a situation that negatively impacts performance?
Yes. You may end up with worse performance, if all components are mindlessly wrapped by React.memo.
It is not needed in many cases. To give it a try with a performance critical component, do some measures first, add memoization and then measure again to see if added complexity was worth it.
React.memo?A memoized component compares old with news props to decide, if to re-render - each render cycle.
A plain component does not care and just renders, after props/state change in a parent.
Take a look at React shallowEqual implementation, which is invoked in updateMemoComponent.
React memo?There are no hard rules. Things, that affect React.memo negatively:
Ad 1: In this case, React.memo cannot prevent a re-render, but had to do additional calculations.
Ad 2: Added comparison cost is not worth it for a "simple" component in terms of render, reconcile, DOM change and side-effect costs.
Ad 3: The more props, the more calculations. You also can pass in a more complex custom comparer.
React.memo?It only checks props, not context changes or state changes from inside. React.memo is also useless, if the memoized component has non-primitive children. useMemo can complement memo here, like:
// inside React.memo component
const ctxVal = useContext(MyContext); // context change normally trigger re-render
return useMemo(() => <Child />, [customDep]) // prevent re-render of children