Use an asynchronous request (or fetch, see below, which is also asynchronous):

function doGET(path, callback) {

    var xhr = new XMLHttpRequest();

    xhr.onreadystatechange = function() {

        if (xhr.readyState == 4) {

            // The request is done; did it work?

            if (xhr.status == 200) {

                // ***Yes, use `xhr.responseText` here***

                callback(xhr.responseText);

            } else {

                // ***No, tell the callback the call failed***

                callback(null);

            }

        }

    };

    xhr.open(GET, path);

    xhr.send();

}



function handleFileData(fileData) {

    if (!fileData) {

        // Show error

        return;

    }

    // Use the file data

}



// Do the request

doGET(/path/to/file, handleFileData);

Or using promises, which are the more modern way to handle callbacks (but keep reading):

function doGET(path, callback) {

    return new Promise(function(resolve, reject) {

        var xhr = new XMLHttpRequest();

        xhr.onreadystatechange = function() {

            if (xhr.readyState == 4) {

                // The request is done; did it work?

                if (xhr.status == 200) {

                    // Yes, use `xhr.responseText` to resolve the promise

                    resolve(xhr.responseText);

                } else {

                    // No, reject the promise

                    reject(xhr);

                }

             }

        };

        xhr.open(GET, path);

        xhr.send();

    });

}



// Do the request

doGET(/path/to/file)

    .then(function(fileData) {

        // Use the file data

    })

    .catch(function(xhr) {

        // The call failed, look at `xhr` for details

    });

Here in 2019, there's no reason to use XHR wrapped in a promise like that, just use fetch:

function doGET(url) {

    return fetch(url).then(response => {

        if (!response.ok) {

            throw new Error(HTTP error  + response.status); // Rejects the promise

        }

    });

}