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1. What is

• A function that is to be executed after another function has finished executing – hence the name call back
• In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions. Functions that do this are called higher-order functions. Any function that is passed as an argument is called a callback function.

2. Why need

JS is an event driven language, instead of waiting for a response before moving on, JS will keep executing while listening for other events.

function first(){
  // Simulate a code delay
  setTimeout( function(){
    console.log(1);
  }, 500 );
}

function second(){
  console.log(2);
}

first();
second();

// 2, 1

The above example shows JS didn’t wait for a response from first() before moving on to execute second()

3. How to

function doHomework(subject, callback) {
  alert(`Starting my ${subject} homework.`);
  callback();
}

doHomework('math', function() {
  alert('Finished my homework');
});

Reference

https://developer.mozilla.org/en-US/docs/Glossary/Callback_function

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