So I'm fetching an API call which I'm then trying to iterate over in order to display as a list. My code so far is: And in fact, the json loads, a sample call is: But I never get the iterating ul elements. I think this is somehow related to how I'm running my useEffect, because in logs I get

ReactJs json map returning undefined after loading

So I’m fetching an API call which I’m then trying to iterate over in order to display as a list. My code so far is:

import React, { useState, useEffect } from "react";
import "./App.css";

function App() {
  const [isLoading, setIsLoading] = useState(true);
  const [trendingMovies, setTrendingMovies] = useState();

  useEffect(() => {
    fetch(
      "https://api.themoviedb.org/3/trending/all/day?api_key=***"
    )
      .then((res) => res.json())
      .then((results) => console.log(results))
      .then((data) => setTrendingMovies(data))
      .catch((error) => console.log(error))
      .then(setIsLoading(false));
  }, [trendingMovies]);

  function Loading() {
    return <h1>Loading...</h1>;
  }

  function DisplayTrendingMovies() {
    return (
      <>
        <p>Trending:</p>
        {console.log(trendingMovies)}
        <ul>
          {trendingMovies &&
            trendingMovies.map((movie) => (
              <li key={movie.results}>{movie.results.original_title}</li>
            ))}
        </ul>
      </>
    );
  }

  return <>{isLoading ? Loading() : DisplayTrendingMovies()}</>;
}

export default App;

JavaScript
​x
 
import React, { useState, useEffect } from "react";
import "./App.css";
​
function App() {
  const [isLoading, setIsLoading] = useState(true);
  const [trendingMovies, setTrendingMovies] = useState();
​
  useEffect(() => {
    fetch(
      "https://api.themoviedb.org/3/trending/all/day?api_key=***"
    )
      .then((res) => res.json())
      .then((results) => console.log(results))
      .then((data) => setTrendingMovies(data))
      .catch((error) => console.log(error))
      .then(setIsLoading(false));
  }, [trendingMovies]);
​
  function Loading() {
    return <h1>Loading...</h1>;
  }
​
  function DisplayTrendingMovies() {
    return (
      <>
        <p>Trending:</p>
        {console.log(trendingMovies)}
        <ul>
          {trendingMovies &&
            trendingMovies.map((movie) => (
              <li key={movie.results}>{movie.results.original_title}</li>
            ))}
        </ul>
      </>
    );
  }
​
  return <>{isLoading ? Loading() : DisplayTrendingMovies()}</>;
}
​
export default App;
​

And in fact, the json loads, a sample call is:

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      "original_language": "en",
      "original_title": "Spider-Man: No Way Home",
      "poster_path": "/1g0dhYtq4irTY1GPXvft6k4YLjm.jpg",
      "video": false,
      "vote_average": 8.2,
      "vote_count": 9831,
      "overview": "Peter Parker is unmasked and no longer able to separate his normal life from the high-stakes of being a super-hero. When he asks for help from Doctor Strange the stakes become even more dangerous, forcing him to discover what it truly means to be Spider-Man.",
      "release_date": "2021-12-15",
      "title": "Spider-Man: No Way Home",
      "id": 634649,
      "adult": false,
      "backdrop_path": "/iQFcwSGbZXMkeyKrxbPnwnRo5fl.jpg",
      "popularity": 9675.798,
      "media_type": "movie"
    },
    {
      "adult": false,
      "backdrop_path": "/qswImgf57wBf2i8Fv6K2O2BdMoe.jpg",
      "genre_ids": [53, 10749, 18],
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      "original_title": "Deep Water",
      "overview": "Vic and Melinda Van Allen are a couple in the small town of Little Wesley. Their loveless marriage is held together only by a precarious arrangement whereby, in order to avoid the messiness of divorce, Melinda is allowed to take any number of lovers as long as she does not desert her family.",
      "poster_path": "/6yRMyWwjuhKg6IU66uiZIGhaSc8.jpg",
      "release_date": "2022-03-18",
      "title": "Deep Water",
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      "vote_count": 1,
      "popularity": 111.471,
      "media_type": "movie"
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      "video": false,
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JavaScript
 
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​

But I never get the iterating ul elements. I think this is somehow related to how I’m running my useEffect, because in logs I get undefined before the json call is completed, the correct json (although it doesn’t iterate/display as ul elements) and then another undefined.

Any pointers?

Answer

I think the problem is with the way fetch api’s promise is handled. .then((results) => console.log(results)) seems to return undefined and the following .then is receiving data as undefined. Please try like below and let me know if it works!

    import React, { useState, useEffect } from "react";

function App() {
  const [isLoading, setIsLoading] = useState(true);
  const [trendingMovies, setTrendingMovies] = useState();

  useEffect(() => {
    fetch(
      "https://api.themoviedb.org/3/trending/all/day?api_key=***"
    )
      .then((res) => res.json())
      .then((data) => setTrendingMovies(data.results))
      .catch((error) => console.log(error))
      .then(setIsLoading(false));
  }, []);

  function Loading() {
    return <h1>Loading...</h1>;
  }

  function DisplayTrendingMovies() {
    return (
      <>
        <p>Trending:</p>
        {console.log(trendingMovies)}
        <ul>
          {trendingMovies &&
            trendingMovies.map((movie) => (
              <li key={movie.name}>{movie.original_title}</li>
            ))}
        </ul>
      </>
    );
  }

  return <>{isLoading ? Loading() : DisplayTrendingMovies()}</>;
}

export default App;

JavaScript
 
    import React, { useState, useEffect } from "react";
​
function App() {
  const [isLoading, setIsLoading] = useState(true);
  const [trendingMovies, setTrendingMovies] = useState();
​
  useEffect(() => {
    fetch(
      "https://api.themoviedb.org/3/trending/all/day?api_key=***"
    )
      .then((res) => res.json())
      .then((data) => setTrendingMovies(data.results))
      .catch((error) => console.log(error))
      .then(setIsLoading(false));
  }, []);
​
  function Loading() {
    return <h1>Loading...</h1>;
  }
​
  function DisplayTrendingMovies() {
    return (
      <>
        <p>Trending:</p>
        {console.log(trendingMovies)}
        <ul>
          {trendingMovies &&
            trendingMovies.map((movie) => (
              <li key={movie.name}>{movie.original_title}</li>
            ))}
        </ul>
      </>
    );
  }
​
  return <>{isLoading ? Loading() : DisplayTrendingMovies()}</>;
}
​
export default App;
​

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Answer