[anderskaseorg]

Parse, don’t validate. If you need a heuristic that accepts non-URL strings as if they were valid URLs, you should convert those non-URL strings to valid URLs so the rest of your code can just deal with valid URLs.

    if (validateURL(url)) {
      return url;
    } else if (validateURL("http://" + url)) {
      return "http://" + url;
    } else {
      return null;
    }

[JadeNB]

I know we're not golfing, but it pains me to see that repetition in the middle. Mightn't we write

    if (!validateURL(url)) {
        url = "http://" + url;
        if (!validateURL(url)) {
            url = null;
        }
    }
    return url;

to snip a small probability of a bug?

[wolfgang42]

I find that branchiness (and mutation of the variable) harder to follow. Personally, I’d just take “parse, don’t validate” to its logical conclusion and go for:

    const parseUrl = url => validateUrl(url) ? url : null;
    return parseUrl(url) || parseUrl('http://'+url) || null;