How about this version in plain ES6/ES2015?

function get(url) {

return new Promise((resolve, reject) => {

const req = new XMLHttpRequest();

req.open('GET', url);

req.onload = () => req.status === 200 ? resolve(req.response) : reject(Error(req.statusText));

req.onerror = (e) => reject(Error(`Network Error: ${e}`));

req.send();

});

}

The function returns a promise. Here is an example on how to use the function and handle the promise it returns:

get('foo.txt')

.then((data) => {

// Do stuff with data, if foo.txt was successfully loaded.

})

.catch((err) => {

// Do stuff on error...

});

If you need to load a json file you can use JSON.parse() to convert the loaded data into an JS Object.

You can also integrate req.responseType='json' into the function but unfortunately there is no IE support for it, so I would stick with JSON.parse().