Better error messages from additionalResolvers

[valcker]

Is your feature request related to a problem? Please describe.

When using additionalResolvers in a declarative way, it is easy to may a typo in a targetSource or targetMethod. At the moment, the error messages due to that are not user-friendly.

For example:

• if I will specify a wrong targetSource name, the message will be "message": "Cannot read property 'api' of undefined".;
• if I'll specify a wrong targetMethod -> "message": "Cannot read property 'type' of undefined".

Describe the solution you'd like:
It would be great to give more user-friendly error messages in that context, i.e.:

• for an incorrect targetSource: additionalResolver error: unknown targetSource ${additionalResolver.targetSource};
• for an incorrect targetMethod: additionalResolver error: targetMethod ${additionalResolver.targetMethod} does not exist on targetSource ${additionalResolver.targetSource}.

Example of an implementation

In packages/config/src/utils.ts

return {
  [additionalResolver.type]: {
    [additionalResolver.field]: {
      selectionSet: additionalResolver.requiredSelectionSet,
      resolve: async (root: any, args: any, context: any, info: any) => {
        const resolverData = { root, args, context, info };
        const methodArgs: any = {};
        for (const argPath in additionalResolver.args) {
          set(methodArgs, argPath, stringInterpolator.parse(additionalResolver.args[argPath], resolverData));
        }
  
        // >>>>>>>>>>>>>>>
        // Basic validation of additionalResolver's parameters
        if (!(additionalResolver.targetSource in context))
          throw new Error(`additionalResolver error: unknown targetSource '${additionalResolver.targetSource}'`);
        if (!(additionalResolver.targetMethod in context[additionalResolver.targetSource].api))
          throw new Error(`additionalResolver error: targetMethod ${additionalResolver.targetMethod} does not exist on targetSource ${additionalResolver.targetSource}`);
        // >>>>>>>>>>>>>>>
  
        const result = await context[additionalResolver.targetSource].api[additionalResolver.targetMethod](
          methodArgs,
          {
            selectedFields: additionalResolver.resultSelectedFields,
            selectionSet: additionalResolver.resultSelectionSet,
            depth: additionalResolver.resultDepth,
          }
        );
        return additionalResolver.returnData ? get(result, additionalResolver.returnData) : result;
      },
    },
  },
  };

[ardatan]

This is available in the newer versions! Thanks for the feedback!