What could be causing a substring that is supposed to match to return "undefined" in JavaScript?

Today, while working with regular expressions in JavaScript (Firefox 3 on Windows Vista), I encountered a peculiar behavior.

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

Despite the regular expression appearing to be correct, there seems to be an issue with the output in the console.log calls.

Surprisingly, Internet Explorer 7 and Chrome both show expected behavior: format[1] returns "%A" (surprisingly, Internet Explorer 7 isn't as buggy as anticipated...)

Could this discrepancy in output be a bug in Firefox, or perhaps a little-known "feature"?

javascriptregexfirefoxfirebug

Answer №1

The reason for this behavior is that console.log() functions similarly to printf(). In console.log(), the first argument serves as a format string, which can then be followed by additional arguments. %A serves as a placeholder in this context. Here's an example:

console.log("My favorite color is %A", "blue"); // My favorite color is "blue"

Refer to console.log() documentation for more information. It appears that %A, along with other placeholders that are not officially documented, have a similar functionality to %o.

Answer №2

It appears as though %A is being interpreted as the value undefined.

Consider adding escape characters to the %A section, that should help resolve the issue.

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