XMLHttpRequest

This is the simplest usage of asynchronous XMLHttpRequest:

var xhr = new XMLHttpRequest();
xhr.open("GET", "/bar/foo.txt", true);
xhr.onload = function (e) {
  if (xhr.readyState === 4) {
    if (xhr.status === 200) {
      console.log(xhr.responseText);
    } else {
      console.error(xhr.statusText);
    }
  }
};
xhr.onerror = function (e) {
  console.error(xhr.statusText);
};
xhr.send(null);

A standard function to read external files:

function xhrSuccess() { 
    this.callback.apply(this, this.arguments); 
}

function xhrError() { 
    console.error(this.statusText); 
}

function loadFile(url, callback /*, opt_arg1, opt_arg2, ... */) {
    var xhr = new XMLHttpRequest();
    xhr.callback = callback;
    xhr.arguments = Array.prototype.slice.call(arguments, 2);
    xhr.onload = xhrSuccess;
    xhr.onerror = xhrError;
    xhr.open("GET", url, true);
    xhr.send(null);
}

Usage:

function showMessage(message) {
    console.log(message + this.responseText);
}

loadFile("message.txt", showMessage, "New message!\n\n");

Example: using a timeout:

function loadFile(url, timeout, callback) {
    var args = Array.prototype.slice.call(arguments, 3);
    var xhr = new XMLHttpRequest();
    xhr.ontimeout = function () {
        console.error("The request for " + url + " timed out.");
    };
    xhr.onload = function() {
        if (xhr.readyState === 4) {
            if (xhr.status === 200) {
                callback.apply(xhr, args);
            } else {
                console.error(xhr.statusText);
            }
        }
    };
    xhr.open("GET", url, true);
    xhr.timeout = timeout;
    xhr.send(null);
}

Usage:

function showMessage (message) {
    console.log(message + this.responseText);
}

loadFile("message.txt", 2000, showMessage, "New message!\n");

Also a way of use:

var request = new XMLHttpRequest();
request.open('GET', 'https://api.cryptonator.com/api/ticker/btc-usd');
request.onreadystatechange = function () {
	if ((request.readyState === 4) && (request.status === 200)) {
		console.log(request);
		console.log(request.responseText);
	}
}
request.send();