Published 17 years 1 month ago • Last updated July 2, 2025 • ⏱️ 2 min read
This is the first time I've looked at the Firefox source, really! :) I wanted to find all the hidden properties Firefox has in Javascript. It was first pointed out to me by DoctorDan on the slackers forums when he found that the RegExp literal had a -1 value for the source in Firefox 2. I then made it my mission to find others because I thought it would be cool.
They seem to be flags within the source (Ronald mentioned this to me at some point too), I'm not sure how they are used internally or within Javascript. In the source code they are given the name tinyid so that's what I'll refer to them from now on.
Here's how to use them:-
(function(){ alert(arguments[-3]) })()
Functions:- CALL_ARGUMENTS = -1, predefined arguments local variable ARGS_LENGTH = -2, number of actual args, arity if inactive ARGS_CALLEE = -3, reference from arguments to active funobj FUN_ARITY = -4, number of formal parameters; desired argc FUN_NAME = -5, function name, "" if anonymous FUN_CALLER = -6 Function.prototype.caller, backward compat
RegExp:- REGEXP_STATIC_INPUT = -1, REGEXP_STATIC_MULTILINE = -2, REGEXP_STATIC_LAST_MATCH = -3, REGEXP_STATIC_LAST_PAREN = -4, REGEXP_STATIC_LEFT_CONTEXT = -5, REGEXP_STATIC_RIGHT_CONTEXT = -6
REGEXP_SOURCE = -1, REGEXP_GLOBAL = -2, REGEXP_IGNORE_CASE = -3, REGEXP_LAST_INDEX = -4, REGEXP_MULTILINE = -5, REGEXP_STICKY = -6;
E4X:- NAMESPACE_PREFIX = -1, NAMESPACE_URI = -2
QNAME:- QNAME_URI = -1, QNAME_LOCALNAME = -2
As I find more I'll add them here, I know strings uses -1 for the length but I'll wait till I find all of them for the string object.