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?

Answer 1

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.