How do I not include duplicate values in a React state array?

Question

I want to make sure that I can't push a duplicated value into an array in a React state. The duplicated value is still going in the array though.

I have tried using .includes but it is not working.

const categoryCheck = (category) => {
  let categories = [...this.state.categories]
  if (this.state.categories === undefined) {
    return
  }
  console.log(categories, category)
  console.log(!categories.includes(category))
  if (!categories.includes(category) === false) {
    categories.push(category)
    console.log('new category', categories)
  } 
}

input: 'cat'

Expected result: ['cat'] Actual result: ['cat', 'cat']

UPDATE: This is my function and this is how I call it. Thanks for all the help!

  uniqueCategories = category => {
    //makes sure that there are no duplicate categories in array
    if (category === undefined) {
      return;
    }
    let categories = new Set(category);
    categories = Array.from(categories);
    console.log(categories);
    return categories;
  };

I call it in another function like this:

  this.setState({
      categories: this.uniqueCategories([
        ...this.state.categories,
        categoryInput
      ])

Answer 1

if (!categories.includes(category) === false) {

is a double negative. Remove the === false.

An alternative is to use a Set for uniqueness since it automatically discards duplicates. Typically, sets offer fast lookup time, but for small amounts of data the performance is probably not much different than array with includes since both structures are rebuilt on every render.

Here's a toy example of using Set:

const App = () => {
  const [categories, setCategories] = React.useState(new Set());
  const [item, setItem] = React.useState("");

  const addCategory = e => {
    if (item.trim()) {
      setCategories(prevState => new Set(prevState).add(item.trim()));
    }
  };

  return (
    <div>
      <input
        onChange={e => setItem(e.target.value)}
        value={item}
      />&nbsp;
      <button onClick={addCategory}>Add Item</button>
      <ul>{[...categories].map(e => <li key={e}>{e}</li>)}</ul>
    </div>
  );
};

ReactDOM.createRoot(document.querySelector("#app"))
  .render(<App />);

<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>
<div id="app"></div>

And an includes example:

const Example = () => {
  const [categories, setCategories] = React.useState([]);
  const [item, setItem] = React.useState("");

  const addCategory = e => { 
    const trimmed = item.trim();

    if (trimmed && !categories.includes(trimmed)) {
      setCategories(prevState => prevState.concat(trimmed));
    }
  };

  return (
    <div>
      <input
        onChange={e => setItem(e.target.value)}
        value={item}
      />&nbsp;
      <button onClick={addCategory}>Add Item</button>
      <ul>{categories.map(e => <li key={e}>{e}</li>)}</ul>
    </div>
  );
};

ReactDOM.createRoot(document.querySelector("#app"))
  .render(<Example />);

<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>
<div id="app"></div>