Why is a matched substring returning "undefined" in JavaScript?

Question

I came across a strange behaviour when doing some regular expressions in JavaScript today (Firefox 3 on Windows Vista).

var str = "format_%A";
var format = /(?:^|\s)format_(.*?)(?:\s|$)/.exec(str);

console.log(format);    // ["format_%A", "%A"]
console.log(format[0]); // "format_undefined"
console.log(format[1]); // Undefined

There's nothing wrong with the regular expression. As you can see, it has matched the correct part in the first console.log call.

Internet Explorer 7 and Chrome both behave as expected: format[1] returns "%A" (well, Internet Explorer 7 doing something right was a bit unexpected...)

Is this a bug in Firefox, or some "feature" I don't know about?

I've never seen the literal matching syntax you're using here. Can you point at some web resource where one can read about it? — PEZ, Jan 11 '09 at 12:34
I think at least a link to the previous almost-the-same question should be provided: http://stackoverflow.com/questions/432493/how-do-you-access-the-matched-groups-in-a-javascript-regex — Rene Saarsoo, Jan 11 '09 at 12:57
@PEZ: https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Regular_Expressions — Robert, Oct 31 '12 at 21:19

score 16 · Accepted Answer · answered Jan 11 '09 at 12:55

16

This is because console.log() works like printf(). The first argument to console.log() is actually a format string which may be followed with additional arguments. %A is a placeholder. For example:

console.log("My name is %A", "John"); // My name is "John"

See console.log() documentation for details. %A and any other undocumented placeholders seem to do the same as %o.

answered Jan 11 '09 at 12:55

Rene Saarsoo

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To be clear, this is not how console.log behaves in Safari's Web Inspector. – eyelidlessness Jan 11 '09 at 18:25
Firefox does not behave like this - this is apparently a firebug specific feature. Has this ever been anything other than a firebug feature? – Eamon Nerbonne Jun 19 '14 at 15:24

score 1 · Answer 2 · answered Jan 11 '09 at 12:35

1

Seems like %A somehow translates into the string undefined.

Try escaping the %A part, I think that will solve the problem.

answered Jan 11 '09 at 12:35

Yuval Adam

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Good catch. It's treated as a URL escape (like %20 and the like) but then %A isn't one of those so gets to be undefined. – PEZ Jan 11 '09 at 12:50
Seems to be an issue with Firebug – PEZ Jan 11 '09 at 13:00

Why is a matched substring returning "undefined" in JavaScript?

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