Merge pull request #482 from sjrd/use-const-in-es2015-example

Use `const` instead of `var` in ES 2015 examples.

Author: sjrd

doc/sjs-for-js/index.md

@@ -8,15 +8,15 @@ Below you can see the same functionality implemented in JavaScript ES6 and Scala
 {% columns %}
 {% column 6 ES6 %}
 {% highlight javascript %}
-var xhr = new XMLHttpRequest();
+const xhr = new XMLHttpRequest();
 
 xhr.open("GET",
   "https://api.twitter.com/1.1/search/" +
   "tweets.json?q=%23scalajs"
 );
 xhr.onload = (e) => {
   if (xhr.status === 200) {
-    var r = JSON.parse(xhr.responseText);
+    const r = JSON.parse(xhr.responseText);
     $("#tweets").html(parseTweets(r));
   }
 };