The answer
Promises use .catch for errors, which catches errors from anywhere in the .then chain. async/await uses try/catch. One handler covers all errors, instead of checking if (err) at every callback level.
Callback Error Handling in JavaScript
How do promises improve error handling over callbacks in JavaScript?
More FAQs in Callback Error Handling in JavaScript
The Node.js convention where the callback's first argument is an error (null if no error). The second argument and beyond are the result. Example: fs.readFile(path, (err, data) => { if (err) ... }).
Check if (err) first and return early. If there is an error, handle it and return. If not, use the result. Always check for the error before using the result to avoid TypeError (result might be undefined).
Because each callback must check if (err) and propagate the error to the caller. In a chain of callbacks, this check is repeated at every level. Promises solve this with a single .catch, and async/await with a single try/catch.
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