What's the Best Way to Transform JavaScript FormData Object Into a Query String?

Let's suppose we have the following FormData object:

const formData = new FormData();

formData.append('name', 'john');
formData.append('langs[]', 'english');
formData.append('langs[]', 'deutsch');
formData.append('t&c', true);

The best way to convert it into a query string is to use the URLSearchParams object like so:

const queryStr = new URLSearchParams(formData).toString();

console.log(queryStr); // output: 'name=john&langs[]=english&langs[]=deutsch&t%26c=true'

If you're wondering why we use toString() here; it's simply because without it, we will get the URLSearchParams object instance whereas with it we get a query string that's properly encoded and suitable for use in a URL.

Please note that URLSearchParams is not supported in any versions of IE. Prior to using it, please make sure you check the browser support for it.