Why must I pass a function to the setter function of 'useState' in React?

I’m handling file uploads using react-dropzone, which calls the following function with a list of files:

    const [files, setFiles] = useState([])

    const onDrop = useCallback(async acceptedFiles => {
    }, [])

I then upload the files using fetch:

    const uploadFiles = async acceptedFiles => {
        acceptedFiles.forEach(async (file) => {
            setFiles(array => [...array, fileState])

            await fetch('/upload', {
                method: 'POST',
                body: data
            }).then(res => {
                // etc

If I replace the setFiles call with just setFiles([...files, fileState]) (not in a function) it breaks, but why is this?

78 thoughts on “Why must I pass a function to the setter function of 'useState' in React?”

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