Question

I have been using the following function to get regular parameters from the current url:

function getURLParameter(name) {
    return decodeURI(
        (RegExp(name + '=' + '(.+?)(&|$)').exec(location.search)||[,null])[1]
    );
}

However, I want to get the GET parameter ‘imgurl’ from a string that will follow the format for the url here.

When I change that function to handle regular strings:

function getURLParameter(name, givenstring) {
    return decodeURI(
        (RegExp(name + '=' + '(.+?)(&|$)').exec(givenstring.search)||[,null])[1]
    );
}

It returns null. I even made a userscript go to the url that I just gave you and I used the first function to try to get that ‘imgurl’ parameter, but it was unable to do so. I also got rid of the colons in that url, but obviously that did nothing.

What is throwing this function off about this string format?

Answer 1

location.search is the query-string part of the current URL. If you want to replace that with an arbitrary string variable givenstring, then you would replace location.search with givenstring, not with givenstring.search. So:

function getURLParameter(name, givenstring) {
    return decodeURI(
        (RegExp(name + '=' + '(.+?)(&|$)').exec(givenstring)||[,null])[1]
    );
}

By the way, your function will misbehave if (for example) the string is foobar=baz&bar=bip and the parameter of interest is bar. It will find the first instance of bar=..., which in this case is bar=baz rather than bar=bip. You can fix that by using (^|&):

function getURLParameter(name, givenstring) {
    return decodeURI(
        (RegExp('(^|&)' + name + '=(.+?)(&|$)').exec(givenstring)||[,,null])[2]
    );
}

(Your function will also misbehave if name has any special characters in it — either special in URLs or special in regexes — but I’m guessing that you have enough control over name that you can make certain that’s not going to be an issue?)