The code snippet you provided seems a bit unclear, so I'll do my best to provide an alternative solution.
To handle async data, you can use either Promises or callbacks. You can chain them together and return the beginning of the Promise.
For example, if your myAsyncCall() function returns a Promise, you can simply do:
return myAsyncCall();
This will return a Promise that resolves once myAsyncCall() is completed. If you need to transform the data, you can chain multiple then() calls:
return myAsyncCall().then(data => transform(data));
This sequence ensures that myAsyncCall() is executed first, followed by any transformations, with subsequent then() calls receiving the latest data.
If myAsyncCall() uses a callback instead, you can convert it to return a Promise:
return new Promise((resolve) => {
myAsyncCall(data => resolve(data));
});
This approach allows myAsyncCall() to resolve first, passing its data to a Promise for further processing.
Based on your recent updates, it appears you want to chain callbacks and Promises effectively in your code.
Let's start with traditional callback chaining:
const a = (start, cb) => cb(start);
const b = (input, cb) => cb(input + 5);
a(1, result =>
b(result, newResult =>
console.log(newResult)
)
);
In this scenario, functions are nested within each other, which can get messy. Some developers prefer breaking it down into separate functions:
const a = (start, cb) => cb(start);
const b = (input, cb) => cb(input + 5);
const onA = result => b(result, onB);
const onB = result => console.log(result);
a(1, onA);
When using Promises, chaining them is done through the then() method. Here's how you can achieve the same functionality with Promises:
const a = start => Promise.resolve(start);
const b = input => Promise.resolve(input + 5);
a(1).then(b).then(result => console.log(result));
If you need to mix callbacks and Promises, one approach is to combine them directly:
const a = start => Promise.resolve(start);
const b = (input, cb) => cb(input + 5);
a(1).then(result => b(result, result => console.log(result)));
To simplify the process, you can create a "promisify" function that converts a callback function to a Promise. Here's a basic implementation:
const promisify = func => (...args) => new Promise(resolve => {
const cb = result => resolve(result);
func.apply(func, args.concat(cb));
});
const a = (start, cb) => cb(start);
const b = (input, cb) => cb(input + 5);
const aPromisified = promisify(a);
const bPromisified = promisify(b);
aPromisified(1)
.then(bPromisified)
.then(result => console.log(result));
This "promisify" function simplifies the conversion of callbacks to Promises, making it easier to work with mixed approaches.
If you require a more advanced "promisify" function that handles callbacks with error handling, you can modify the function as follows:
const promisify = (func, withErr = false) => (...args) => new Promise((resolve, reject) => {
const handler = (err, result) => {
if(withErr) {
err ? reject(err) : resolve(result);
} else {
resolve(err);
}
}
func.apply(func, args.concat(handler));
});
const aCallback = (start, cb) => cb(start);
const bCallback = (input, cb) => cb(input === 3 && 'cannot be 3', input + 5);
const a = promisify(aCallback);
const b = promisify(bCallback, true);
a(1).then(b).then(r => console.log(r));
a(3).then(b).catch(err => console.error(err));