>From the TC39 notes: > DaveH: What if there are duplicate top-level bindings? > MarkM: Top-level let should behave as much as possible the same as > concatenated scripts.
It's my claim that it is impossible to reconcile all of the following three goals in a way that is even remotely sane: 1. Top-level bindings are aliased as data properties of the global object. 2. Top-level bindings allow redefinition of existing properties of the global object. 3. Top-level let/const have a temporal dead-zone. Abandoning (3) essentially means abandoning a reasonable "const" semantics for the toplevel (and future hostile to guards), so we don't want to do that. (For V8, we had long struggles with defining and implementing a const for the (current notion of) top-level that actually makes any sense. We failed.) Hence, I would like to suggest another, more moderate idea for a global scope reform: treat the global object similar to a module instance objects. That is: - Top-level bindings (other than var) are reflected as accessor properties on the global object. - They are defined on the global object before the script starts executing. - Their getters/setters simply delegate to the bound variables. - For const bindings, there are no setters. - These properties are non-configurable. - Unlike a module instance object, the global object is extensible (multiple scripts!). This disallows lexical bindings to redefine existing non-configurable properties of the global object (including ones reflecting bindings from earlier scripts), which is checked before the script starts executing. Likewise, let/const-defined properties cannot later be deleted from the global object. Apart from that, not much should change in practical terms. You should pretty much be able to replace var with let. At the same time, static scoping actually is maintained, and there is no issue with temporal dead zones. Moreover, concatenating scripts would not affect the meaning or validity of bindings, AFAICS. Regarding scope pollution, I think this is best solved by making the global object be a fresh empty object that has the object containing all primordials as its prototype. Overriding is allowed according to the standard defineProperty semantics. No special rules necessary. /Andreas _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
