Koa.js backend unable to access response body in callback function

Question

I have a backend created using koa.js which contains a callback function to set the response body. In particular, the middleware function for the backend looks like the follow

exports.middlewareFunction = async (ctx) => {
      
     let data = 11;
     someAsyncFunc(data, function (result) => {
          ctx.body = result;
     });
}

The program produces an error that the route is unable to be accessed. My guess on the error is that since the callback in someAsyncFunc is executed asynchronously, this causes ctx.body to be set after the main flow has already been executed. In particular, the call to the middleware has already returned before ctx.body is set in the callback. If my understanding is correct, is there a way to fix it, so that I can still modify ctx.body asynchronously but at the same time allowing it to be returned in the response?

Answer 1

I'd put this in a comment, but my rep is too low. There are a couple problems within the code you provided and it seems like you might be newer to JavaScript, or at least async functions, so I just wanted to provide the MDN async function docs and also the MDN Promise docs to help you further understand.

To address your provided code:

exports.middlewareFunction = async (ctx) => {
      
     let data = 11;
     someAsyncFunc(data, function (result) => {
          ctx.body = result;
     });
}

1. Why is your middlewareFunction async, but there is no await within your function? If someAsyncFunc is async, you should be using await.
2. If your someAsyncFunc is async, you should consider returning a value from that and get rid of the callback. By using await and returning the data as a promise with someAsyncFunc, you then don't have to worry about the issue you are having and can safely manipulate your ctx.body by using await correctly on your async function.

proposed example fix:

exports.middlewareFunction = async (ctx) => {
      
     let data = 11;
     const result = await someAsyncFunc(data); // no need for callback
     ctx.body = result;
     // rest of code

}