How Do Block-Level Functions Behave in ES6 Under Different Strict Mode and Web Extensions Settings?

Understanding the Nuanced Semantics of Block-Level Functions in ES6

The introduction of block-level functions in ES6 brought forth complexities that warrant thorough examination. Despite the notion that they follow the rules of block hoisting and scope restriction in strict mode, the optional "web extensions" provision creates additional intricacies.

To clarify these semantics, consider the following table, which outlines the behavior of block-level function declarations under various conditions:

Mode	Web Extensions	Visible Outside Block	Hoisted	TDZ
Non-Strict, No Extensions	No	Yes, as a var	Yes, to Function and Block	Undefined Before Block
Strict, No Extensions	No	No	Yes, to Block	N/A

| Non-Strict, With Extensions | Yes | Yes, as a var | Yes, to Function and Block | Undefined Before Function Declaration |

| Strict, With Extensions | Yes | Yes, as a var | Yes, to Block | N/A |

The key distinction lies in the concept of "strict mode." In the context of block-level functions, strict mode refers to the strictness of the function (or script) in which the block containing the function declaration occurs, not the strictness of the function itself.

In non-strict mode with web extensions, a function declaration in a block is deemed "sane" if its name does not conflict with formal parameters or lexically declared variables. In such cases, an additional var declaration is hoisted to the enclosing function scope, and the function object is assigned to this variable upon function declaration evaluation.

This behavior mirrors the following code structure:

function enclosing(...) {
    var compat₀ = undefined;
    {
        let compat₁ = function compat(…) { … };
        compat₀ = compat₁;
    }
}

Hence, the function becomes visible outside the block, similar to a var declaration. However, an additional block-scoped binding exists within the block.

In strict mode, regardless of web extensions, block-level functions are hoisted to the top of the block, and their visibility is restricted within that block.

It's crucial to note that the aforementioned semantics may not always translate directly to practical implementations. Inconsistencies among ES6 implementations are not uncommon, highlighting the importance of understanding the ES6 specification itself, irrespective of implementation variations.

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