May function declarations appear inside statements in JavaScript?

Question

Please consider the official ECMAScript specification as the source for your answer, and not a document published by a specific browser vendor. (I am aware of Mozilla extend

Answer 1

No, it's invalid. Function declarations can only appear as "source elements", which are either in the global scope or immediately within another function definition, outside all other statements. From the ECMA-262 spec:

FunctionBody : SourceElements

Program : SourceElements

SourceElement : Statement | FunctionDeclaration

There is no other production in the grammar that allows a FunctionDeclaration.

Only function expressions are allowed to be part of a statement:

MemberExpression : FunctionExpression

...

Statement : ExpressionStatement

Edit: There was a related discussion on another question recently. See the comments on this answer - earlier, I too thought that this could be valid but the grammar makes it clear that it is invalid.