GET with query string with Fetch in React Native

Your first thought was right: just add them to the URL.

Remember you can use template strings (backticks) to simplify putting variables into the query.

const data = {foo:1, bar:2};

fetch(`https://api.parse.com/1/users?foo=${encodeURIComponent(data.foo)}&bar=${encodeURIComponent(data.bar)}`, {
  method: "GET",
  headers: headers,   
})