GET with query string with Fetch in React Native

I did a small riff on Mark Amery's answer that will pass Airbnb's eslint definitions since many teams seem to have that requirement these days.

function objToQueryString(obj) {
  const keyValuePairs = [];
  for (let i = 0; i < Object.keys(obj).length; i += 1) {
    keyValuePairs.push(`${encodeURIComponent(Object.keys(obj)[i])}=${encodeURIComponent(Object.values(obj)[i])}`);
  }
  return keyValuePairs.join('&');
}

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,   
})

Here's an es6 approach

const getQueryString = (queries) => {
    return Object.keys(queries).reduce((result, key) => {
        return [...result, `${encodeURIComponent(key)}=${encodeURIComponent(queries[key])}`]
    }, []).join('&');
};

Here we're taking in a queries object in the shape of key: param We iterate and reduce through the keys of this object, building an array of encoded query strings. Lastly we do a join and return this attachable query string.

Short answer

Just substitute values into the URL like this:

const encodedValue = encodeURIComponent(someVariable);
fetch(`https://example.com/foo?bar=${encodedValue}`);

Longer answer

Yes, you just need to add the query string to the URL yourself. You should take care to escape your query string parameters, though - don't just construct a URL like

`https://example.com/foo?bar=${someVariable}`

unless you're confident that someVariable definitely doesn't contain any &, =, or other special characters.

If you were using fetch outside of React Native, you'd have the option of encoding query string parameters using URLSearchParams. However, React Native does not support URLSearchParams. Instead, use encodeURIComponent.

For example:

const encodedValue = encodeURIComponent(someVariable);
fetch(`https://example.com/foo?bar=${encodedValue}`);

If you want to serialise an object of keys and values into a query string, you could make a utility function to do that:

function objToQueryString(obj) {
  const keyValuePairs = [];
  for (const key in obj) {
    keyValuePairs.push(encodeURIComponent(key) + '=' + encodeURIComponent(obj[key]));
  }
  return keyValuePairs.join('&');
}

... and use it like this:

const queryString = objToQueryString({
    key1: 'somevalue',
    key2: someVariable,
});
fetch(`https://example.com/foo?${queryString}`);