Says that Firefox Nightly does not support let and const.

[grssam]

Am I missing something ? Firefox has supported let and const since very long ago.

[ruanyf]

What is the test's result?

https://github.com/ruanyf/es-checker/blob/master/lib/api.js#L2

[grssam]

When I try running the following code on any page, no alert comes:

try {
  new Function("'use strict'; let a; const b = 2;");
} catch (ex) { alert(ex) }

[ruanyf]

if no alerts, it means your browser supports let/const.

try {
  (new Function("'use strict'; let a; const b = 2;"))();
} catch (ex) { 
  alert(ex) 
}

[grssam]

http://jsfiddle.net/bpnzduzq/ works. It alerts NaN

The only thing is that you have to specifically specify js version as 1.7 or above.

[ruanyf]

fixed it.