Callbacks2026-04-06

My Solution: Exercism Callbacks solution

Callbacks are functions passed into other functions so they can be called later.

Introduction

A callback must match the signature expected by the function that calls it.

const squareLength = 5;

function applyToSquare(callback) {
  return callback(squareLength);
}

function areaOfSquare(number) {
  return number * number;
}

applyToSquare(areaOfSquare); // => 25

You can pass callbacks as named functions, anonymous functions, or arrow functions.

Instructions

Build a small library for the grocer's API:

• onSuccess() should call notify with { message: 'SUCCESS' }.
• onError() should call notify with { message: 'ERROR' }.
• orderFromGrocer(query, onSuccessCallback, onErrorCallback) should forward all arguments to order.
• postOrder(variety, quantity) should call orderFromGrocer with the query object and the shared callbacks.

Keep the callbacks reusable so the same success and error handlers can be passed around.

Solution

import { notify } from './notifier';
import { order } from './grocer';

export function onSuccess() {
  return notify({ message: 'SUCCESS' });
}

export function onError() {
  return notify({ message: 'ERROR' });
}

export function orderFromGrocer(query, onSuccessCallback, onErrorCallback) {
  return order(query, onSuccessCallback, onErrorCallback);
}

export function postOrder(variety, quantity) {
  return orderFromGrocer({ variety, quantity }, onSuccess, onError);
}

Tests

onSuccess();
// => notify called with { message: 'SUCCESS' }

onError();
// => notify called with { message: 'ERROR' }

orderFromGrocer(
  { variety: 'pear', quantity: 12 },
  exampleSuccessCallback,
  exampleErrorCallback,
);
// => order called with the query and callbacks

postOrder('peach', 100);
// => order placed for 100 peaches

Mental Model

• A callback is just a function value passed to another function.
• The caller decides when the callback runs.
• The callback must accept the arguments the caller will provide.
• Named functions are easier to reuse when the same behavior is needed in multiple places.
• Wrapping order inside orderFromGrocer keeps the rest of the codebase isolated from API changes.