How Do ES6 Block-Level Functions Behave in Strict and Non-Strict Modes, with and without Web Extensions?

Block-Level Functions in ES6: Semantic Precision

The ES6 specification introduces block-level functions, allowing function declarations within blocks. Understanding their semantics is critical for effective programming.

Block-Level Function Behaviors

Setting	Visible Outside Block	Hoisted	Hoisting Scope	Temporal Dead Zone
Non-strict, No Web Extensions	No	No	N/A	N/A
Strict, No Web Extensions	No	Yes	Block Level	Yes
Non-strict, With Web Extensions	Yes	Yes	Function Level	Before Block
Strict, With Web Extensions	Yes	Yes	Function Level	Before Block

Strict Mode and Block-Level Functions

"Strict mode" in this context refers to the strictness of the function or script in which the block containing the function declaration appears, not the function itself. "web extensions" apply only to sloppy (non-strict) code with "sane" function appearance (no name collisions).

In pure ES6 (without "web extensions"), function declarations in blocks have consistent semantics regardless of strictness. They are hoisted to the top of the block and behave like regular function declarations.

Web Compatibility Semantics

With "web extensions," sloppy mode introduces additional semantics. A function declaration within a block is hoisted to the top of the enclosing function as a var declaration, and the function object is assigned to this variable when the function declaration is evaluated.

This results in two bindings for the same identifier: one function-scoped (visible outside the block) and one block-scoped (visible only within the block). The function-scoped binding is initialized with undefined until the function is declared, at which point it is assigned the function object.

Before the function declaration is encountered in block execution, the function-scoped binding is undefined and accessing it will throw an exception.

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