Initialize an array in React component render (in terms of performance)

Question

It is known that we should not use anonymous functions in render because they will be re-create on each render. But what about an array of objects like this:

// name props is passed in so it seems like there is a point in initializing the array inside the component
const Component = ({ name }) => {
  // let's imagine there is a lot of large objects
  const bigArray = [{id: 1, name}, {id: 2, name}];

  return <AnotherComponent data={bigArray} />;
}

I suppose that arrays (and objects inside) are stored by reference so when the component is re-rendered, the interpreter sees that this is the same reference and won't re-create the array. Is my assumption correct?

Is the array dynamic or fixed? What's the point of recreating it when you could just have it set globally? — kemicofa
– kemicofa, Commented Oct 10, 2019 at 12:45

Andres · Accepted Answer · 2019-10-10 12:46:18Z

3

No, it's not the same reference. You're creating new array with new objects inside on each render. If it's completely static, you can extract it from your function to keep the same ref:

const name = 'some-name';
const bigArray = [{id: 1, name}, {id: 2, name}];

const Component = () => (
   <AnotherComponent data={bigArray} />;
);

But in your case this array generated from prop. You can use useMemo() hook to optimize it:

const Component = ({ name }) => (
  const bigArray = useMemo(
    () => [{id: 1, name}, {id: 2, name}],
    [ name ]
  );

   <AnotherComponent data={bigArray} />;
);

answered Oct 10, 2019 at 12:46

Andres

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Maxime Pie · Accepted Answer · 2019-10-10 12:51:30Z

1

Unfortunately when using a React.Component or React.FunctionnalComponent it seems that every single change will re-render the complete array.

We can consider using a React.PureComponent as a workaround of this issue, more infos here : https://codeburst.io/react-array-re-render-performance-ee23f34c5d66

answered Oct 10, 2019 at 12:51

Maxime Pie

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Comments

Horatiu Jeflea · Accepted Answer · 2019-10-10 12:44:52Z

0

It is better if you move bigArray then (considering it does not change) to a more global state.

// let's imagine there is a lot of large objects
const bigArray = [{id: 1, name}, {id: 2, name}];

// name props is passed in so it seems like there is a point in initializing the array inside the component
const Component = ({ name }) => {
  return <AnotherComponent data={bigArray} />;
}

answered Oct 10, 2019 at 12:44

Horatiu Jeflea

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1 Comment

HMR Over a year ago

Where does name come from? You can't initialize the big array outside the Component. I think the useMemo answer is best.

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