differences between API calls with Koa and Express - express

Question

Previously I worked with Express a bit and now learning Koa.
In Express, when I did a simple get method. The following code works
.get('/applications', (req, res, next) => {
const getTable = `SELECT * FROM applicationtable`
db.query(getTable, values, (err, result) => {
if (err) {
console.log(err)
}else {
res.json(result)
}
})
})
However when I tried to do the same thing in Koa with
.get('/applications', (ctx, next) => {
const getTable = `SELECT * FROM applicationtable`
db.query(getTable, values, (err, result) => {
if (err) {
console.log(err)
}else {
ctx.body = result
}
})
})
It returns a 404 error. I have to explicitly wrap the above one in Koa with a promise and put async await then only it will work, as shown below:
.get('/applications', async(ctx, next) => {
const getTable = `SELECT * FROM applicationtable`
const item = await new Promise(function(resolve, reject) {
db.query(getTable, (err, result) => {
if (err) {
reject(err)
}else {
resolve(result)
}
})
})
ctx.body = item;
})
My question is , why in Express the code does not have to be explicitly wrapped in async await? In what way Koa is behaving differently than Express with the examples here?

Answer 1

Koa is all designed around promises. The request handler itself is async which means it returns a promise and various things happen in the framework when that promise resolves. As such, you can't freely mix plain asynchronous callbacks with promises.
In the case of your second code block, your request handler returns (which resolves the async promise) BEFORE your asynchronous callback gets called and thus before you set ctx.body. So, Koa goes to process the request (thinking you are done), but ctx.body hasn't been set yet so it figures this must be a 404.
Wrapping your async operation in a promise and using await with it, then chains it into the main request handler promise so that promise doesn't resolve until your asynchronous operation is done (what you want).
Bottom line, use promises that are linked into the request handler (either with await or by returning a promise) for all your asynchronous operations in the request handler.
Here's a nice little example in the Koa developer's guide: https://github.com/koajs/koa/blob/master/docs/guide.md#async-operations
Express, on the other hand, does not do anything when your request handler returns. It doesn't do anything until you either call next() to continue looking for matches with other route handlers or call res.send() or some similar API call that sends a response so you're free to do that in a plain asynchronous callback like you show.
This is part of the core design difference between Express and Koa.