GraphQL-codegen fails to load GraphQLSchema

Question

So i have a problem loading a GraphQl schema using gql-codegen. When I try to generate, it fails with 404 error, however schema is available at http://localhost:8000/graphql below is the error.

 Cannot convert undefined or null to object
      TypeError: Cannot convert undefined or null to object
      at Function.entries (<anonymous>)

I've tried to replace locahost with 127.0.0.1 in my npm file but i got the same result. Any help is much appreciated. Thanks in advance!

Answer 1

After some debugging I found that the error comes from ../node_modules/@graphql-tools/url-loader/cjs/index.js from the following method:

  class UrlLoader {
       ...
   handleSDL(pointer, fetch, options) {
       for (const [key, value] of Object.entries(options.headers)) {
                    if (key.startsWith(':')) {
                      // omit http2 headers
                      isHttp2 = true;
                    } else {
                      headers[key] = value
                    }
                  }   
        return new value_or_promise_1.ValueOrPromise(() => fetch(pointer, {
                method: defaultMethod,
                headers: typeof (options === null || options === void 0 ? void 0 : options.headers) === 'function' ? options.headers() : options === null || options === void 0 ? void 0 : options.headers,
            }))
    ... }

The problem was that the options.headers property might be undefined or null, which cannot be iterated using Object.entries(). What solved my problem was adding a check to ensure that options.headers is defined before iterating through its entries:

if (options.headers) {
  for (const [key, value] of Object.entries(options.headers)) {
    // ...
  }
}

And similarly, for the fetch call, I added a check to ensure that options.headers is defined before passing it to the fetch method.:

return new value_or_promise_1.ValueOrPromise(() => fetch(pointer, {
  method: defaultMethod,
  headers: typeof options?.headers === 'function' ? options.headers() : options?.headers || {},
})); 

Then after running gql-gen again the error was gone and everything worked as expected.