If you can post the entire code snippet then it will be helpful to answer. Though you're possibly doing something like this.

Initially, it is declared as an empty array [], But there is a possibility that somewhere in your code you're setting the state like this.

  setUserFavorites(someDataFromAPICallORPostOperation).

Here, Your someDataFromAPICallORPostOperation is possibly undefined.

You can solve this in two ways.

1. Default to an empty array while setting the state.

  setUserFavorites(someDataFromAPICallORPostOperation ?? []).

2. Check for the undefined before calling the .some function.

userFavorites?.some(function) // Using optional chaining

or

userFavorites && userFavorites.some(function)

EDIT - Based on the code mentioned in the commented link.

The array push operation doesn't return the updated array, Instead, it returns the number of elements in the updated array.

This can be solved as

    function addFavoritesHandler(favoriteMeetup) {
        setUserFavorites(prevUserFavorites => {
            return [...prevUserFavorites, favoriteMeetup];
        });
    }

Explanation, Earlier you were doing something like this.

    function addFavoritesHandler(favoriteMeetup) {
        setUserFavorites(prevUserFavorites => {
            return prevUserFavorites.push(favoriteMeetup);
        });
    }

Here you will set the state to a number instead of an array as push will return the number of elements in that array. Hence, while doing userFavorites.some it throws an error as you are setting the userFavorites to a number instead of an array.