Note: more details on let at block scoped bindings.

— Dave Herman 2010/08/20 15:37

Strong consensus for let as the new var, with the same syntax but block scope.

We want let to share the same “temporal dead zone” as const: no use before declaration. Note however that let does not require an initializer; the default initial value is undefined.

As with const, let x; let x is an error (again ignoring initializers).

We think let should behave like const when the environment frame is an object and not declarative (at top level in global code, and in eval called from global code): you get an error if there exists a property of that object with the same name when the let is evaluated. So in browsers, let name = 42 will throw (due to window.name).

Open issues:

• What error should be thrown in the previous paragraph?
• Should a = []; for (let i = 0; i < 3; i++) a.push(function () { return i; }); print(a[0]()) print 3 or 0?
  • Some say yes (Mark did a lambda coding).
  • Others say no (and Mozilla’s JS1.7 in Rhino and SpiderMonkey implement this, but these could be changed and users might be happier).
  • Jon Zeppieri pointed out a big flaw: for (let i=0;i<N;i++){...} differs from let i=0; for (;i<N;i++){...}.
• Same example but using for (let i in o) – here everyone agrees that in this form, we want fresh binding per iteration. Same for for-of (see iterators).
• Bad old grammar and semantics in ES1-5 that permit for (var i = E in o) where E is evaluated before the loop begins.
  • We are stuck with this closure bug for var.
  • We agree for let and const that the initializer = E should be forbidden grammatically.

— Brendan Eich 2009/09/24 21:10