The use-local-storage hook returns the real value only after mounting. Is it a bug?

How use-local-storage hook works

By default, the use-local-storage hook retrieves the value from local storage in useEffect only after the component is mounted. It is implemented this way to avoid hydration mismatches if the value is used in the output markup of the component.

Example:

• value during server-side rendering: dark (default value)
• value at initial render: dark (default value)
• value after mounting: light or dark depending on the value in local storage (value from local storage)

import { useLocalStorage } from '@mantine/hooks';

function Demo() {
  const [value, setValue] = useLocalStorage<'light' | 'dark'>({
    key: 'color-scheme',
    defaultValue: 'dark',
  });

  return <div>{value}</div>;
}

Reading value in first render

If your application does not have server-side rendering or you do not use the value in the output markup of the component, you can read the value from local storage in the first render. To do that, set the getInitialValueInEffect: false option:

Example:

• value during server-side rendering: dark (default value)
• value at initial render: light or dark depending on the value in local storage (value from local storage)
• value at subsequent renders: light or dark depending on the value in local storage (value from local storage)

import { useLocalStorage } from '@mantine/hooks';

function Demo() {
  const [value, setValue] = useLocalStorage<'light' | 'dark'>({
    key: 'color-scheme',
    defaultValue: 'dark',
    getInitialValueInEffect: false,
  });

  return <div>{value}</div>;
}