Change all snippets from var to const

snippets/measure-time-taken-by-function.md

@@ -4,7 +4,7 @@ Use `performance.now()` to get start and end time for the function, `console.log
 First argument is the function name, subsequent arguments are passed to the function.
 
 ```js
-var timeTaken = (f,...args) => {
+const timeTaken = (f,...args) => {
   var t0 = performance.now(), r = f(...args);
   console.log(performance.now() - t0);
   return r;