What is a Callback in JavaScript?

Simply put, callbacks are functions passed as an argument to other functions to be used once an event has occurred or a task is finished. You can think of an on click event listener attached to a DOM element or a callback provided for a network request:

// This function will only run when the event occurs
const onClick = () => console.log('☝️');

document.addEventListener('click', onClick);

Why do we need callbacks? While JavaScript runs sequentially, (from top to bottom) callbacks lets you execute code in an asynchronous manner. Note that you can also define an anonymous function as a callback, you don't necessarily need to declare it.

// Anonymous function passed as a callback
document.addEventListener('click', () => {
    ...
});

50 JavaScript Interview QuestionsAnd their answers explainedHere are 50 JavaScript interview questions that often come up during a technical interview. In one place, one resource to learn from.Learn More

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Resources:

• Callback functions in JavaScript