When creating web applications it’s common to create links. Sometimes, links contain query string parameters. Google uses the ?q= query string parameter for search queries. An issue can arise when query string values contain characters that have special meanings.

For instance, the & character separates different query string items, while = separates keys and values like ?q=peas. If these characters are part of query string values, like ?q=peas&carrots, carrots can be interpreted as the name of the next item in the query string, rather than part of the value of q.

Here’s the problem demonstrated:

new URLSearchParams('https://example.com?q=peas&carrots')
  .has('carrots'); // true, but we want this to be false


You can make the characters safe for a url by url encoding the query string values (as well as the keys, if necessary). With JavaScript we can do this as follows:

  // "https://example.com?q=peas%26carrots"

new URLSearchParams(`https://example.com?q=${encodeURIComponent('peas&carrots')}`)
  .has('carrots'); // false, which is what we want

The & was transformed to %26, making it safe for a url.

For non-JavaScript cases follow the guidance of your specific use case. For example, with ASP.NET (both .NET Framework and .NET Core) you can use WebUtility.UrlEncode(string).


Some frameworks and libraries may do this automatically. But, if it’s not happening automatically it should be handled manually. If you’re not sure, try it out with peas&carrots!