👀 Views: 71 💬 Answers: 1 📅 Created: 2025-06-14

react hooks arrays javascript

I'm getting frustrated with I'm relatively new to this, so bear with me. I'm writing unit tests and I'm working through a tutorial and I'm stuck on something that should probably be simple. Hey everyone, I'm running into an issue that's driving me crazy. I'm experiencing a `TypeError: want to read properties of undefined (reading 'map')` when trying to merge two arrays using the spread operator in my React component that utilizes hooks. I'm trying to combine a state array with another array fetched from an API. Here's a simplified version of my code: ```javascript import React, { useEffect, useState } from 'react'; const MyComponent = () => { const [data, setData] = useState([]); const [extraData, setExtraData] = useState([]); useEffect(() => { fetch('https://api.example.com/data') .then(response => response.json()) .then(fetchedData => setExtraData(fetchedData)); }, []); const handleMerge = () => { const mergedData = [...data, ...extraData]; console.log(mergedData); }; return ( <div> <button onClick={handleMerge}>Merge Data</button> </div> ); }; export default MyComponent; ``` Initially, `data` is an empty array, and `extraData` is also an empty array until the API call resolves. When I click the merge button, I see the TypeError in the console. I checked that `extraData` is being set correctly after the fetch, but I'm not sure why I'm getting this behavior. I've also tried adding a check to ensure that `extraData` is an array before the merge, but it didn't help. Any insights into what might be causing this scenario or how I could handle potential undefined values more gracefully would be appreciated. I'm using React 17.0.2. Any ideas what could be causing this? Is there a better approach? This is happening in both development and production on Debian. What would be the recommended way to handle this? I recently upgraded to Javascript stable. What's the best practice here? I'm working with Javascript in a Docker container on macOS. For context: I'm using Javascript on Windows 11.