The following will log the JSON response from the API to the console if the request was successful, otherwise it will log the error.
var xhr = new XMLHttpRequest();
xhr.open('GET', 'https://api.twitch.tv/kraken', true);
xhr.setRequestHeader('Client-ID', '...');
xhr.onload = function(data){
console.log(data);
};
xhr.onerror = function(error){
console.log(error.target.status);
};
xhr.send();