Verifying the age of a remote JSON file in Javascript without needing to download the full file

I'm working on a Phonegap app that needs to compare a local JSON file with the latest version available remotely. If the remote version is newer, I want to update the local JSON.

To optimize bandwidth usage, I am looking for a way to perform this version check without downloading the entire remote data. Perhaps by just checking the headers...

In my search for a solution, I came across discussions about using the "HTTP ETag header" in threads like this one

Does anyone have suggestions on how to implement something like this using pure Javascript?

Thank you in advance.

javascriptjsonhttp

Answer №1

When your server responds with a 304 - Not modified status, you have the option to check for an update using the following function:

function checkForUpdate(url, callback)
{
    var request = new XMLHttpRequest();
    request.open('HEAD', url);
    request.onreadystatechange = function() {
        if (this.readyState == this.DONE) {
            if (this.status === 200) {
              // Data is new -- consider making another request to get updated information
            } else if (this.status === 304) {
              // Data has not been modified
            } else {
              // An error occurred while fetching data.
            }
        }
    };
    request.send();
}

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