let and const support

ES6 let and const declarations are now supported when targeting ES3 and ES5.

Const

const MAX = 100;
++MAX; // Error: The operand of an increment or decrement
       //        operator cannot be a constant.

Block scoped

if (true) {
  let a = 4;
  // use a
}
else {
  let a = "string";
  // use a
}
alert(a); // Error: a is not defined in this scope