Why does the statement if ( !condition ) { console.log(condition) } display true [duplicate]

Question

I want to make a String method, which accepts a RegExp and a callback, then splits String by RegExp, and inserts callback's return in split array. In short, it would do something like this:

"a 1 b 2 c".method(/\d/, function ($1) { return $1 + 1; })
    => [a, 2, b, 3, c]

In case the String doesn't match the RegExp, it should return an array, like this:

"a b c d e".method(/\d/, function ($1) { return $1 + 1; })
    => ["a b c d e"]

I wrote this code, but it doesn't work as I thought:

String.prototype.preserveSplitReg = function(reg, func) {

    var rtn = [], 
        that = this.toString();
    if (!reg.test(that)) {
        console.log(reg, that, reg.test(that));
        return [that];
    }

    ...
}

The console.log should be called ONLY when the String doesn't match reg, right? But sometimes it logs (reg, that, true). That troublesome String and reg were:

"See <url>http://www.w3.org/TR/html5-diff/</url> for changed elements and attributes, as well as obsolete elements and"
/<url>.*?<\/url>/g

console logs true. I can't figure out why. Any help would be highly appreciated.

Answer 1

This is due to a bug in Javascript RegEx engine (ECMAScript3)

Basically a regex with the g modifier doesn't reset correctly, so multiple test() calls toggle between true and false.

• See also this answer: https://stackoverflow.com/a/3827500/548225 for more details.