Higher-Order Functions and Callbacks

Pass behavior as an argument

A higher-order function accepts a function, returns one, or both. A callback is a function supplied for another operation to invoke. Callbacks may run synchronously or asynchronously; the word callback alone does not tell you when they execute.

function transformEach(values, transform) {
  const result = [];
  for (const value of values) result.push(transform(value));
  return result;
}
const marks = [10, 20, 30];
const doubled = transformEach(marks, value => value * 2);
console.log(doubled); // [20, 40, 60]
console.log(marks);   // [10, 20, 30]

transformEach controls traversal while the callback controls the transformation. It invokes the callback immediately during its loop. By contrast, a timer callback runs later.

Return a specialized function

function atLeast(minimum) {
  return value => value >= minimum;
}
const isPassing = atLeast(40);
console.log([35, 40, 82].filter(isPassing)); // [40, 82]

The returned callback closes over minimum. This is useful when one generic operation needs a small configured rule.

Pass the function, not its result

setTimeout(showMessage, 100) passes a function for later execution. setTimeout(showMessage(), 100) calls it immediately and passes whatever it returned. For arguments, wrap the call in a function: setTimeout(() => showMessage('Ready'), 100).

Callback contracts

Document the arguments, return value, execution timing and error behavior. A callback used by map must return the mapped value; one used by forEach has its return value ignored. A synchronous try/catch around timer registration cannot catch an exception thrown later inside the timer callback.

Practice

1. Pass a square callback to transformEach; [2,3] should become [4,9].
2. Build a predicate for scores at least 80.
3. Log before and after a synchronous array callback, then repeat with setTimeout to compare ordering.
4. Explain why transformEach does not itself decide whether to double, square or format a value.