Error-First Callbacks

Node.js conventions use error-first callbacks: the first argument is an error (or null), and the second is the result. Always check the error first!

function readFile(filename, callback) {
  if (!filename) {
    callback(new Error("No filename"), null);
  } else {
    callback(null, "File contents here");
  }
}

readFile("data.txt", (err, data) => {
  if (err) { console.log("Error:", err.message); return; }
  console.log(data);
});

FINAL CHALLENGE

Write a function divide that takes two numbers and a callback. If the divisor is 0, call back with an error message "Cannot divide by zero". Otherwise, call back with null and the result. Test with 10 and 2, print the result.

Need a hint?

Call callback(null, a / b) for the success case. 10/2 = 5.

Expected Output:

5