How Do ES6 Block-Level Function Semantics Differ in Strict and Non-Strict Modes, and How Do Web Extensions Affect Them?

Understanding the Semantics of ES6 Block-Level Functions

Introduction
With the advent of ES6, block-level function declarations became a valuable addition to the language. Despite initial assumptions, the precise semantics of these functions encompass a wider spectrum, including distinctions between strict and non-strict modes and browser compatibility considerations.

Semantics
The table below summarizes the key aspects of block-level function semantics:

Execution Environment	Visible Outside Block	Hoisted to Top of Block	TDZ
Non-strict, no web extensions	Yes, like var	Yes	None
Strict, no web extensions	No	Yes	None
Non-strict, web extensions	Yes, like var	Yes	None
Strict, web extensions	No	Yes, twice (function and block)	Function-scoped binding is undefined before declaration

Strict Mode Implications
The concept of "strict mode" in this context refers to the [[Strict]] internal slot of the function object, not the strictness of the function itself. Therefore, the code snippet involving a function declaration with "use strict" within a non-strict surrounding code is still considered "non-strict."

Web Extensions
The "web extensions" apply only to non-strict (sloppy) code with "sane" function statement appearances. In sloppy mode with web compatibility semantics, a function declaration within a block is handled as follows:

1. The function declaration is hoisted to the top of the lexical block.
2. A var declaration is hoisted to the enclosing function, initialized to undefined.
3. When the function declaration is evaluated, the function object is assigned to the function-scoped variable.

In essence, this behavior results in two separate bindings with the same name, one block-scoped and the other function-scoped.

Conclusion
While block-level functions in ES6 offer extended functionality, understanding their precise semantics, including the interplay between strict modes and web compatibility, is crucial to ensure proper usage and avoid potential pitfalls.

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