Skipping renders, and memoization in React

In many situations, a React component will re-render when it doesn't have to.
If the result of rendering would be exactly the same as the previous time the component rendered, skipping that render (reconciliation) step entirely is desirable.

Class based components

shouldComponentUpdate

In class components, the method shouldComponentUpdate allows this.
It's a lifecycle method that is called before render(). The method returns a boolean. That boolean tells React if render() can be skipped.

When true, the render() will be executed like it normally would.

When false, that tells React it can skip executing the render().

shouldComponentUpdate() is called with the next props and the next state. This allows complex logic where the current props/state are compared to the previous props/state in order to determine if the output would be different and thus, the component should update.

By default, shouldComponentUpdate() returns true.
Not specifying this method at all is the same as

shouldComponentUpdate(nextProps, nextState) {
 return true
}

Don't rely on this to prevent rendering altogether. It might work right now, but it can lead to bugs and is likely to change in the future. Instead, treat it as a hint to React that tells it "you can safely skip rendering this, the result will be the same as the previous result anyway".

The logic in shouldComponentUpdate can quickly get very complex and is prone to mistakes.
Before you know it, that method will look something like this

shouldComponentUpdate(nextProps, nextState) {
  const propsComparison = this.props.a !== nextProps.a && this.props.b !== nextProps.b && this.props.c !== nextProps.c && this.props.d !== nextProps.d
  const stateComparison = this.state.one !== nextState.one && this.state.two !== nextState.two && this.state.three !== nextState.three
 return propsComparison && stateComparison
}

😢 I just wanted to check if any props or state changed, why is that so hard?

React.PureComponent

React.PureComponent does exactly that! 😎

PureComponent performs a shallow comparison of props and state (by using Object.is).
This reduces the chance that you’ll skip a necessary update (e.g. when you add a new prop).

Unless you are confident you need a custom shouldComponentUpdate, prefer PureComponent.

That means these two snippets are equivalent

class Driver extends React.Component {
  shouldComponentUpdate() {
    // a shallow comparison of all the props and state
  }
  render() {
    <p>{this.props.name}</p>;
  }
}

class Driver extends React.PureComponent {
  render() {
    <p>{this.props.name}</p>;
  }
}

Function components

When trying to apply that same optimization to function components instead of class based ones, a problem rears its head. Function components can't really skip that render step. The function component (which is really just a function) is either executed or it isn't.

This is where memoization helps.

Memoization is basically technobabble for remembering something for later.

React can't just remember pieces of data for later, it can remember entire components.

React.memo

React.memo does this!

What the previous two examples were for class based components, React.memo is for function components.

Instead of skipping the render-step like in class based components, React.memo will reuse the last rendered result instead of calculating a new result.

// the function component
const Driver = function(props) {
  return <p>{props.name}</p>;
};
// exporting the memoized function component
export default React.memo(Driver);

• Initial render of the memoized Driver component with props { name: "Charles Leclerc" }
  • The function component renders <p>Charles Leclerc</p>.
• The props change to { name: "Daniel Ricciardo" }
  • The components renders <p>Daniel Ricciardo</p>
• Something else changes that triggers an update to our Driver component
  • React.memo sees that the props haven't changed.
  • Instead of calculating the render result, React uses the previous result: <p>Daniel Ricciardo</p>

By default React.memo is comparable to React.PureComponent as it performs a shallow comparison of all props (by using Object.is again).
If you want more control and be in charge of that comparison, React.memo accepts a second argument, a comparison function. This makes it comparable to shouldComponentUpdate in class based components.

The comparison function also returns a boolean.
That boolean tells React if it should use the previous result of the component instead of calculating a new one.

When false, the function component will be executed like it normally would.

When true, the function component will not be executed and the previous result will be used instead.

Watch out! This is the opposite from shouldComponentUpdate!

The comparison function is called with the previous props and the next props. This allows complex logic where the current props are compared to the previous props in order to determine if the output would be different and thus, the remembered result/memo of the component should be used.

// the function component
const Driver = function(props) {
  return <p>{props.name}</p>;
};
// the custom comparison function
const comparisonFn = function(prevProps, nextProps) {
  return prevProps.name === nextProps.name;
};
// exporting the memoized function component
export default React.memo(Driver, comparisonFn);

To extend the parallels with class based components:
Unless you are confident you need a custom comparison function, prefer the default behaviour.

Examples

In this demo there is a toplevel component with 2 pieces of state, a count and an unusedCount. Which, as the name suggests, will remain unused 🙃.
You can increment the count and the unusedCount through buttons.

The top component has 4 children, all of them will display the count and how many times that child component rendered.

The components that have one of the optimizations bescribed above will only render when count is updated. The other ones will also render when the unusedCount is updated.

direct codesandbox link

React.memo vs React.useMemo

While React.memo is a higher-order component as it accepts a component and returns the new/memoized component.

React.useMemo is a hook(which is a function). It accepts a function and returns the memoized return value of the function you passed.

React.useMemo

const memoizedValue = React.useMemo(() => computeExpensiveValue(a, b), [a, b]);

React.useMemo accepts a function as first argument. The value this function returns is the value that React.useMemo will return. It will only be calculated again if it has to. React.useMemo will return the memoized/remembered value if it doesn't.

You tell React.useMemo if it should recalculate that result through the second argument, an array. The value the passed function returns will only be calculated again if something in that array of dependencies changes. Not passing anything would cause that value to be calculated every time the component renders (and causes the function to run).

Every value used inside the function you passed should be included in the dependencies array.
This will prevent a lot of unintended behaviour.

The React team has created an ESLint package, eslint-plugin-react-hooks that is designed to warn you when breaking the rules of hooks. The dependencies array being complete is checked by a rule in that package called exhaustive-deps.

Example

import React from 'react';

function calculatePodiums(name) {
  // very expensive calculation
  return numResult;
}

const Driver = function(props) {
  const numOfPodiums = React.useMemo(() => calculatePodiums(props.name), [
    props.name
  ]);
  return (
    <div>
      <p>My name is: {props.name}</p>
      <p>I drive for: {props.team}</p>
      <p>I have been on the podium {numOfPodiums} times</p>
    </div>
  );
};

• Initial render of our Driver component with props { name: "Kimi Räikkönen", team: "Ferrari" }
  • The function component calculates numOfPodiums and renders using the result of that calculation.
• The props change to { name: "Kimi Räikkönen", team: "Alfa Romeo Racing" }
  • React.useMemo sees nothing in the dependencies array has changed and does not recalculate numOfPodiums
  • The memo/remembered value for numOfPodiums is used.
• The props change again to { name: "Antonio Giovinazzi", team: "Alfa Romeo Racing" }
  • React.useMemo sees something changed in the dependencies array and calculates numOfPodiums
  • The freshly calculated value is used.

Bonus: React.useCallback

This is a shortcut for a specific React.useMemo usage.

React.useMemo returns a memoized value

React.useCallback returns a memoized function

🤔 But a value can totally be a function!

Correct!
That means these two snippets are equivalent

const memoizedFunction = React.useMemo(function() {
  return function doTheThing(a, b) {
    // do the thing
  }
}
}, [a, b])

⬆ This memoizes the value the first argument (a function) returns, which is a function called doTheThing.

const memoizedFunction = React.useCallback(function doTheThing(a, b) {
    // do the thing
  }
}, [a, b])

⬆ This memoizes the first argument, which is a function called doTheThing.

Like React.useMemo, the second argument is an array of dependencies.
The function React.useCallback returns will only change when something in that array changes.