1071877 - Support ES6 block level function declarations in strict mode

Status: RESOLVED DUPLICATE of bug 1071646

(Core :: JavaScript Engine, defect)

Description

[Jason Orendorff [:jorendorff]]

In SM strict mode code, a FunctionDeclaration can't be appear immediately inside a Block. You get a SyntaxError.

In ES6 draft rev 27, in strict mode code, a FunctionDeclaration can occur in a Block. It's a block-local function, hoisted to the top of the block.

Strict mode only because of web compatibility. Bug 585536 comment 48 has some of the dream-crushing details.

Comment 1

[Jason Orendorff [:jorendorff]]

In short, this just works how you'd expect. It's pretty great.

But to make it concrete, an example:

    1   "use strict";
    2   for (let x of obj) {
    3       f();
    4       function f() { /* ... code that refers to f and x ... */ }
    5   }

Each time through the loop, as we enter the Block starting with the curly brace at the end of line 2, we create a new function f and bind it to a new block-scoped variable 'f', then evaluate the StatementList inside the Block.

Because the binding 'f' is initialized before the StatementList starts to run, there's no TDZ. It's OK to call f() on line 3, ahead of the declaration of f on line 4.

All this is more or less there in ES6 draft rev 27, though it's beautifully obfuscated.