React Hooks setState of an element in an array

Question

How can I update a single element in a state array? Here is the code that I am currently using:

const Cars = props => {
  const [cars, setCars] = React.useState(["Honda","Toyota","Dodge"])

  const handleClick1 = () => { setCars[0]("Jeep") }
  const handleClick2 = () => { setCars[1]("Jeep") }
  const handleClick3 = () => { setCars[2]("Jeep") }

  return (
    <div>
      <button onClick={handleClick1}>{cars[0]}</button>
      <button onClick={handleClick2}>{cars[1]}</button>
      <button onClick={handleClick3}>{cars[2]}</button>
    </div>
  )
};

When I click one of the rendered buttons, I get Uncaught TypeError: setCars[0] is not a function at handleClick1.

I know how to do this in a React Class, but how can I do this with React Hooks?

Matt Oestreich · Accepted Answer · 2019-12-15 00:35:38Z

I suggest you map through your cars in order to render them - this is just overall a million times easier. From there you can apply an onClick handler to each button..

Furthermore, you should not mutate state like you are - always make a copy of state first, update the copy, then set your new state with the updated copy.

Edit: one thing that slipped my mind before was adding a key to each item when you are mapping over an array. This should be standard practice.

const { useState } = React;
const { render } = ReactDOM;

const Cars = props => {
  const [cars, setCars] = useState(["Honda", "Toyota", "Dodge"]);

  const updateCars = (value, index) => () => {
    let carsCopy = [...cars];
    carsCopy[index] = value;
    setCars(carsCopy);
  };

  return (
    <div>
      {cars && cars.map((c, i) => 
        <button key={`${i}_${c}`} onClick={updateCars("Jeep", i)}>{c}</button>
      )}
      <pre>{cars && JSON.stringify(cars, null, 2)}</pre>
    </div>
  );
};

render(<Cars />, document.body);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.12.0/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.11.0/umd/react-dom.production.min.js"></script>

imiro · Accepted Answer · 2019-12-14 20:48:06Z

0

I think you should correct these lines to spot the source of error

const handleClick1 = () => { setCars[0]("Jeep") }

into

const handleClick1 = () => { cars[0]="Jeep"; setCars(cars); }

answered Dec 14, 2019 at 20:48

imiro

1677 bronze badges

2 Comments

skyboyer Over a year ago

you are mutating state variables. It's "no-no" in React world. Re-rendering probably will run anyway(it least for class-based component setState re-render even if no changes), but once you pass this state as a prop to some child you man go in trouble.

imiro Over a year ago

yes i see your point, but considering the that is not the numero uno issue in question its some times better to keep it simple: Uncaught TypeError

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