How Do Block-Level Function Declarations Behave in ES6 Strict and Non-Strict Modes?

Block-Level Function Semantics in ES6

Block-level function declarations were introduced in ES6, prompting questions about their semantics.

Core Semantics

Non-strict mode	Strict mode
Hoisted and visible outside of the block	Not visible outside of the block

Web Extension Behavior

In browsers, web extension semantics apply to block-level functions in non-strict mode:

Web Extensions	Hoisted?	Visible Outside of Block?	TDZ?
Yes	To both the block and function	Yes, as a var declaration	Undefined prior to block execution

Strict Mode Clarification

"Strict mode" in the context of block-level functions refers to the strictness of the function in which the block containing the function declaration occurs. The following example demonstrates this:

// Non-strict surrounding code

{
    function foo() { "use strict"; }
}

This code is considered "strict mode" for function foo.

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