index.md

---

title: Callback function
slug: Glossary/Callback_function
page-type: glossary-definition

---

A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

Here is a quick example:

function greeting(name) {
  alert(`Hello, ${name}`);
}

function processUserInput(callback) {
  const name = prompt("Please enter your name.");
  callback(name);
}

processUserInput(greeting);

The above example is a {{glossary("synchronous")}} callback, as it is executed immediately.

Note, however, that callbacks are often used to continue code execution after an {{glossary("asynchronous")}} operation has completed — these are called asynchronous callbacks. A good example is the callback functions executed inside a .then() block chained onto the end of a promise after that promise fulfills or rejects. This structure is used in many modern web APIs, such as fetch().

See also

• Callback on Wikipedia