Foundations Updated 2026-09 View as Markdown

Callbacks

Passing a function to another function — and typing its signature so the parameters come out inferred.

Callbacks in JavaScript are functions that are passed as arguments to other functions. This is a very important feature of asynchronous programming, and it enables the function that receives the callback to call our code when it finishes a long task, while allowing us to continue the execution of the code.

For example:

let callback = function() {
    console.log("Done!");
}

setTimeout(callback, 5000);

This code waits 5 seconds and prints out “Done!” when the 5 seconds are up. Note that this code will not work in the interpreter because it is not designed for handling callbacks.

It is also possible to define callbacks as anonymous functions, like so:

setTimeout(function() {
    console.log("Done!");
}, 5000);

Like regular functions, callbacks can receive arguments and be executed more than once.

setTimeout(function() {
    console.log("Done!");
}, 5000);