approach_1.md

Approach 1. Error-First Callbacks

The first approach is a widespread pattern in Node.js. It is one of the ways that we can deal with errors that are generated during asynchronous processes in Node.js. A callback argument is passed as an additional argument to a function, which will be executed both in the case where an operational error has occurred, and when the operation completes successfully.

In the error case, the callback is executed with an Error object as its only argument. In the success case, the callback is executed with null as its first argument and the result as its second argument. The callback can then check if the first argument is not null and proceed accordingly.

Example

const applyToInteger = (func, integer, callback) => {
  if (typeof func !== "function") {
    callback(new TypeError("Invalid argument: First argument is not a function"));
  }
  else if (!Number.isInteger(integer)) {
    callback(new TypeError(`Invalid argument: Second argument ${integer} is not an integer`));
  }
  else {
    callback(null, func(integer));
  }
};

const applyAndPrintResult = (func, integer) => {
  applyToInteger(func, integer, (err, result) => {
    if (err) {
      console.log("Sorry, result could not be calculated:");
      console.log(err.message);
    } else {
      console.log("Result successfully calculated:");
      console.log(`Applying ${func.name} to ${integer} gives ${result}`);
    }
  });
};

Using this function in the REPL:

> applyAndPrintResult(function double (n) { return 2 * n; }, 2)
Result successfully calculated:
Applying double to 2 gives 4

> applyAndPrintResult(function increment (n) { return n + 1; }, -4)
Result successfully calculated:
Applying increment to -4 gives -3

> applyAndPrintResult({}, 2)
Sorry, result could not be calculated:
Invalid argument: First argument is not a function

> applyAndPrintResult((n) => n, 2.3)
Sorry, result could not be calculated:
Invalid argument: Second argument 2.3 is not an integer

Guidance

• This is the convention for handling errors when using callbacks for asynchronous control flow in Node. Try not to deviate from this pattern when writing async callback-based code.
• Structurally similar to returning errors in that it requires checks in the calling code.
• Maintain consistent interfaces; if a function requires a callback, do not also throw or return errors to the caller (these methods will be covered in approaches 2 and 3).

Trying it out

If you want to try this out yourself, complete the exercise in exercises/approach-1. Test your solutions by running npm run ex-1.