Is JavaScript a Context Free Language?

Question

This article on how browsers work explains how CSS is context free, while HTML is not. But what about JavaScript, is JavaScript context free?

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Answer 1

I'm pretty certain JS is not context free — given an arbitrary code artefact, you cannot necessarily determine its exact meaning without knowing its context.

The first example that comes to mind is {} — does this represent an empty object literal or an empty statement block? It's impossible to decide without context, but because the language allows semicolons to be omitted from statements ending in '}' (as do most languages with C-like syntax) it may also be undecidable with context! Consider {x: {}} — this could be an object literal with the "x" field containing an empty object, or a statement block with a labelled sub-statement (where the label is 'x' and the sub-statement is {}). Perhaps the language specification has some rules for selecting the correct interpretation in such scenarios, but in any case the language does not appear to be context-free, judging by these examples alone.

JavaScript's 'automatic semicolon insertion' feature certainly doesn't help in distinguishing expressions and statements.

Here's another one to think about: function x() {} — what does this do? If it's a statement, it declares a new hoisted variable 'x' with this function as its value. If it's an expression, it simply evaluates to a function which has an upvalue 'x' bound to the same function (for self-reference).