How to Store JSON Response in a Global Variable Using XMLHttpRequest in JavaScript

Описание: Learn the effective method to store JSON responses from a file into a global variable in JavaScript using XMLHttpRequest. This guide explains the process in simple terms, helping you implement it in your projects.
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How to Store JSON Response in a Global Variable Using XMLHttpRequest in JavaScript

JavaScript has become an essential part of web development, enabling interactive and dynamic features in our applications. One common scenario developers encounter is fetching data from JSON files. However, a frequent source of confusion arises when trying to store this data in a global variable for later use. If you have ever found yourself grappling with why your JSON response isn't saving correctly, you’re in the right place.

The Problem

Let’s say you have defined a global variable and are attempting to read data from a JSON file using XMLHttpRequest. Unfortunately, when you try to console log this global variable after assigning it the fetched data, you find it returning undefined. This can be frustrating, yet it’s a common mistake in asynchronous programming in JavaScript.

Here’s a brief look at the initial code that may be causing you trouble:

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Key Issue

The primary problem here lies in the asynchronous nature of the XMLHttpRequest. When you call console.log(myArr) at the end of the script, the request to fetch the data might not have completed yet, leading to the undefined result.

The Solution

You can address this issue straightforwardly by ensuring that your code correctly utilizes the asynchronous capabilities of JavaScript while assigning the fetched data to a global variable. Here’s an effective way to implement this:

Step-by-Step Implementation

Declare a Global Variable: At the start of your JavaScript code, define a global variable that will hold the data.

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Set Up the XMLHttpRequest: Create an instance of XMLHttpRequest and open the request to fetch the JSON data.

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Handle the Response: Use the onload event to handle the response when it is received. Inside this function, check if the request was successful and then parse the response.

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Send the Request: Finally, send the request.

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Use the Global Variable: Remember not to use the myArr variable until you are sure the data has been fetched, which can be done after the onload function.

Example Code

Here is how your entire JavaScript code could look like after these modifications:

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Summary

By understanding the asynchronous nature of XMLHttpRequest and properly managing when to access your global variable, you can successfully store and utilize JSON data in your JavaScript applications. Remember, working with responses in JavaScript often requires a solid grasp of callbacks and event handling!

Now, you're equipped to tackle this common JavaScript situation with confidence.