Set default header for every fetch() request

Question

Is it possible, using the fetch API, to set default headers for every single request?
What I want to do is set an Authorization header whenever there is a json web token in the localStorage. My current solution is to set the headers with this function:

export default function setHeaders(headers) {
    if(localStorage.jwt) {
        return {
            ...headers,
            'Authorization': `Bearer ${localStorage.jwt}`
        }
    } else {
        return headers;
    }
}

Setting the headers in a fetch request would then look like this:

return fetch('/someurl', {
        method: 'post',
        body: JSON.stringify(data),
        headers: setHeaders({
            'Content-Type': 'application/json'
        })
    })

But there has to be a better way to do this. I'm currently developing a React/Redux/Express app if that is of any help.

Answer 1

Creating a fetch wrapper could solve your problem:

function updateOptions(options) {
  const update = { ...options };
  if (localStorage.jwt) {
    update.headers = {
      ...update.headers,
      Authorization: `Bearer ${localStorage.jwt}`,
    };
  }
  return update;
}

export default function fetcher(url, options) {
  return fetch(url, updateOptions(options));
}

You also get the added benefit of being able to switch your request client easily for all the calls in your application if you decide you like Axios or other package better. And you can do other things like check if options.body is an object and add the 'Content-Type: application/json header.