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
– PEZ, Commented Jan 11, 2009 at 12:34
I think at least a link to the previous almost-the-same question should be provided: stackoverflow.com/questions/432493/… — Rene Saarsoo
– Rene Saarsoo, Commented Jan 11, 2009 at 12:57

Rene Saarsoo · Accepted Answer · 2009-01-11 12:55:28Z

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, 2009 at 12:55

Rene Saarsoo

14k10 gold badges63 silver badges91 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

eyelidlessness Over a year ago

To be clear, this is not how console.log behaves in Safari's Web Inspector.

Eamon Nerbonne Over a year ago

Firefox does not behave like this - this is apparently a firebug specific feature. Has this ever been anything other than a firebug feature?

Yuval Adam · Accepted Answer · 2009-01-11 12:35:07Z

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, 2009 at 12:35

Yuval Adam

166k95 gold badges318 silver badges406 bronze badges

2 Comments

PEZ Over a year ago

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 Over a year ago

Seems to be an issue with Firebug

Collectives™ on Stack Overflow

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

2 Answers 2

2 Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related