How Do ES6 Block-Level Function Declarations Behave Regarding Hoisting, Visibility, and Strict Mode?

Precise Semantics of Block-Level Functions in ES6

In ES6, block-level function declarations bring about new semantics compared to traditional function declarations. This article delves into the precise behavior of these functions, addressing questions on their visibility, hoisting, and the concept of "strict mode" in the context of block-level functions.

Table of Semantics

Based on the provided information, the behavior of block-level functions can be summarized in the following table, considering strict and non-strict modes with and without web extensions:

Mode	Visible Outside of Block?	Hoisted?	TDZ?
Non-strict, No Web Extensions	No	Hoisted to block	No
Strict, No Web Extensions	No	Hoisted to block	No
Non-strict, With Web Extensions	Yes *	Hoisted twice (to function and block)	No
Strict, With Web Extensions	Yes *	Hoisted twice (to function and block)	No

* Visible as a function-scoped variable, but also has a block-scoped binding.

"Strict Mode" Definition Clarification

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. It does not refer to the strictness of the function being declared within the block.

Web Extensions and Sloppy Code

The concept of "web extensions" only applies to sloppy (non-strict) code. In this context, a function declaration inside a block in sloppy mode has the following behavior with web extensions:

1. Function declaration is hoisted to the top of both the block and the enclosing function.
2. A function-scoped variable with the same name as the function is also hoisted.
3. When the function declaration is evaluated, the function is assigned to the function-scoped variable.

Conclusion

Understanding the semantics of block-level functions in ES6 is essential for writing clear and predictable code. This article has aimed to clarify the exact behavior of these functions, highlighting the potential complexities introduced by web extensions and the concept of strict mode. By adhering to these semantics, developers can avoid potential pitfalls and create robust code that leverages the benefits of block-level functions in ES6.

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