Callbacks and Asynchronous Work

A callback is a function passed to another function to be called later or at a chosen point. Callbacks are common in event listeners, timers, array methods, and APIs.

function processName(name, callback) {
  const normalized = name.trim();
  callback(normalized);
}

processName("  Asha  ", (value) => {
  console.log(`Hello, ${value}`);
});

Not every callback is asynchronous. Array methods such as map invoke their callback immediately during the transformation. Timer callbacks and many I/O callbacks run later.

Callback-based asynchronous code#

Older APIs often use a callback to provide either an error or a result. Nested asynchronous callbacks can become difficult to read when each step depends on the last; this is sometimes called callback hell.

Promises provide a more composable representation of a future result and are covered next. Many modern browser APIs and Node.js APIs use promises.

Do not assume a timer is exact#

setTimeout(fn, 0) does not execute immediately. It schedules a callback to run after the current JavaScript stack finishes and the runtime can process the task. The actual delay can be longer than requested.

Practice#

Write a function that accepts a value and a callback. Use it synchronously first, then use setTimeout to observe that a callback scheduled for later runs after the current statements.