Block-level function declarations Web Legacy Compatibility bug

Allen Wirfs-Brock allen at
Sat Jul 18 02:34:56 UTC 2015

Good catches...

On Jul 17, 2015, at 5:12 PM, Kevin Gibbons wrote:

> In ECMAScript 2015, functions declared at the top level of function bodies and of scripts are var-like and functions declared in blocks are let-like (visible only in that block), generally speaking. This breaks some previously common patterns using block-level function declarations, which were not a part of ECMAScript 5 but were commonly supported in browsers, albeit with varying semantics. Thus, the ECMAScript 6 spec has an extension (annex B.3.3) which is intended to minimize this breakage by allowing function declarations in blocks to *also* be var-like when possible. This means that they create an additional binding in the containing function's scope which is assigned when execution reaches the block function declaration.
> However, B.3.3 looks for functions that permit these additional bindings among the containing function's VarScopedDeclarations, which essentially consists of top-level function declarations and any var declarations. Importantly, it does not include any function declarations inside of blocks, and so will never be applied.
> Unfortunately, the function body's LexicallyScopedDeclarations will also fail to contain function declarations inside of blocks. No existing static semantic rule will include the desired function declarations; changing the spec to accommodate this may require substantial additions beyond the scope of this email.

Right, see fix in 

> Also, I observe that these additional compatibility steps are only performed for function bodies, not scripts. I'm not sure if this is intentional, but it's a bit counterintuitive: `(function(){ { function f(){} } f(); })();` continues to work with no reference errors, but `{ function f(){} } f();` does not.

See fix in 

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