Refactoring a class based component to a Functional component

Question

TL;DR: I am trying to refactor my class-based App component to a Functional component and I am having some unexpected results with the useEffect Hook.

I am fairly new to React and the version of my app to search for movies with the OMDB API was a class based one. It was working well so I wanted to practice some more on working with hooks and context before venturing into redux.

In my class component I had the following code:

componentDidMount = () => {
    window.onscroll = this.handleScroll;
  };

  handleScroll = () => {
    if (!this.state.movies.loading) {
      if (getScrollDownPercentage(window) > 0.8) {
        const nextPage = this.state.movies.page + 1;
        this.searchMovies(this.state.movies.searchValue, nextPage);
        this.setState({
          ...this.state,
          movies: {
            ...this.state.movies,
            page: nextPage,
          },
        });
      }
    }
  };

  searchMovies = (str, page = 1) => {
    const existingMovies =
      this.state.movies.data.length && this.state.movies.searchValue === str
        ? this.state.movies.data
        : [];
    this.setState({
      ...this.state,
      movies: {
        ...this.state.movies,
        loading: true,
      },
    });
    axios
      .get(`${process.env.REACT_APP_ENDPOINT}s=${str}&page=${page}`)
      .then((response) => {
        if (response.data.Response === "True") {
          this.setState({
            ...this.state,
            movies: {
              ...this.state.movies,
              loading: false,
              searchValue: str,
              data: [...existingMovies, ...response.data.Search],
            },
          });
        }
      });
  };

After the refactor I have following code which feels less clean (so the opposite of what I was hoping to achieve)

The moment I reach the end of the page I get a non-stop stream of axios calls switching between page 2 and 3

If I'm not wrong it's not working because setState is running asynchronously and I am getting constant updates on my page.

  useEffect(() => {
    window.onscroll = () => {
      handleScroll();
    };
    if (searchValue) {
      console.log(page);
      searchMovies(searchValue, page);
    }
    return () => {
      window.removeEventListener("scroll", handleScroll);
    };
  }, [searchValue, page]);

  const handleScroll = () => {
    if (!loading && searchValue) {
      if (getScrollDownPercentage(window) > 0.7) {
        setState({
          ...state,
          movies: {
            ...state.movies,
            page: page + 1,
          },
        });
      }
    }
  };

  const searchMovies = (str, page = 1) => {
    const existingMovies = data.length && prevSearchValue === str ? data : [];
    setState({
      ...state,
      movies: {
        ...state.movies,
        loading: true,
      },
    });
    axios
      .get(`${process.env.REACT_APP_ENDPOINT2}s=${str}&page=${page}`)
      .then((response) => {
        if (response.data.Response === "True") {
          setState({
            ...state,
            movies: {
              ...state.movies,
              loading: false,
              searchValue: str,
              data: [...existingMovies, ...response.data.Search],
            },
          });
        } else {
          setState({
            ...state,
            movies: {
              ...state.movies,
              loading: false,
              error: true,
            },
          });
        }
      });
  };

I hope I included everything (and also not too much) to make my clear my problem. Looking out for some React wizards.

Edit: specified problem a little bit more and put it in bold

Answer 1

Given your reply, I think what you'd want to do multiple useEffects : one for searchMovies and one for the scroll setup.

useEffect(() => {
  window.onscroll = () => {
     handleScroll();
  };
  return () => {
    window.removeEventListener("scroll", handleScroll);
  };
}, []);

useEffect(() => {
  if (searchValue) {
    console.log(page);
    searchMovies(searchValue, page);
  }
}, [searchValue, page])

Answer 2

A quick fix will be use an empty array as the second argument to avoid infinite loop

useEffect(() => {
    //your code
  },[]);