The answer
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.
Callback Error Handling in JavaScript
Why is error handling with callbacks repetitive 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).
Check if (err) in each callback. If there is an error, call the outer callback with the error and return: if (err) { callback(err); return; }. This passes the error up the chain.
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