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Function names with property lookups in JScript 5.8 / WSH

I was looking at Windows Script Host code that runs in JScript 5.8 and I noticed this construct:

function String.prototype.foo() { ... }

I can't find any documentation about it! I'm fairly sure this is not legal in EcmaScript, so it has to be a Microsoft-only extension. Does anyone have any links explaining its design and semantics?

Solution

  • The JScript team released a draft of a document on their blog called "JScript Deviations from ES3" that describes this syntax among others.

    From section 4.4:

    JScript permits function names to be qualified. This allows for function foo.prototype() to be syntactic sugar for foo.prototype = function (){} Note that the object on LHS of the '.' must already be defined.

    On this post from IEBlog about IE9 and ECMAScript 5 compatibility, they mention that this syntax is not supported by IE9 standards mode. I would imagine the same holds true for later versions of IE as well.