Bug 1277479: Use more ES6 features in Y.js; r=fitzgen ; DONTBUILD

MozReview-Commit-ID: G8f9iSL43Jw

--HG--
extra : rebase_source : 391ea220595c4b17f279e3f00538268641a7eae0
extra : amend_source : a3852f238df4cb618a42e7d102faec1ddac6f697

js/src/Y.js

@@ -2,13 +2,13 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-// The Y combinator, applied to the factorial function
+// The Y combinator, applied to the factorial function.
 
 // Return the function that is the fixed point of f.
-var Y = f => (x => f(v => x(x)(v)))
+const Y = f => (x => f(v => x(x)(v)))
                (x => f(v => x(x)(v)));
 
 // The factorial function is the fixed point of this:
-var f = fac => n => (n <= 1) ? 1 : n * fac(n - 1);
+const f = fac => n => (n <= 1) ? 1 : n * fac(n - 1);
 
-print("5! is " + Y(f)(5));
+print(`5! is ${Y(f)(5)}`);