Javascript variable scope

Question

Javascript variable scope

let preGameRequest = new XMLHttpRequest();
let preGameData;

preGameRequest.open("GET", "/matches/live.json");
preGameRequest.onload = function() {
    preGameData = JSON.parse(preGameRequest.responseText);
}
preGameRequest.send();
console.log(preGameData);   // issue arises here

I've encountered a problem with my code. I declared preGameData as a global variable and attempted to store the data from the '/matches/live.json' file into it. However, when I attempt to log preGameData outside of its scope (like in the code section), it returns 'undefined'. Strangely enough, logging preGameData from within the scope works perfectly fine. I'm quite puzzled by this behavior and unsure of what exactly is causing it.

javascript angularjs xmlhttprequest

Answer 1

Answer №1

Here's a revised version of the code that should function correctly:

preGameRequest.onload = function() {
    preGameData = JSON.parse(preGameRequest.responseText);
    console.log(preGameData);
}

The key explanation: is due to the fact that the preGameRequest.send() function operates asynchronously.

When the console tries to run console.log() (as shown in your example), the HTTP response has not been received yet, resulting in the variable being undefined.

However, by placing console.log() inside the handler function (like in my provided example), it will execute only when the response is fully received (within the onload() handler). This ensures that the variable preGameData will be defined and accessible at that point.

Answer 2

Here's a revised version of the code that should function correctly:

preGameRequest.onload = function() {
    preGameData = JSON.parse(preGameRequest.responseText);
    console.log(preGameData);
}

The key explanation: is due to the fact that the preGameRequest.send() function operates asynchronously.

When the console tries to run console.log() (as shown in your example), the HTTP response has not been received yet, resulting in the variable being undefined.

However, by placing console.log() inside the handler function (like in my provided example), it will execute only when the response is fully received (within the onload() handler). This ensures that the variable preGameData will be defined and accessible at that point.

Javascript variable scope

Answer №1

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