What is a callback function?
JavaScript, Function · Oct 3, 2021

A callback function is a function passed as an argument to another function, which is then invoked inside the outer function. Callback functions are often executed once an event has occurred or a task has completed.
Synchronous callbacks
A synchronous callback is a callback function that is executed immediately. The function passed as the first argument to Array.prototype.map()
is a great example of a synchronous callback:
const nums = [1, 2, 3];
const printDoublePlusOne = n => console.log(2 * n + 1);
nums.map(printDoublePlusOne); // LOGS: 3, 5, 7
Asynchronous callbacks
An asynchronous callback is a callback function that is used to execute code after an asynchronous operation has completed. The function executed inside Promise.prototype.then()
is a great example of an asynchronous callback:
const nums = fetch('https://api.nums.org'); // Suppose the response is [1, 2, 3]
const printDoublePlusOne = n => console.log(2 * n + 1);
nums.then(printDoublePlusOne); // LOGS: 3, 5, 7
Written by Angelos Chalaris
I'm Angelos Chalaris, a JavaScript software engineer, based in Athens, Greece. The best snippets from my coding adventures are published here to help others learn to code.
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