function tenSquared() {
  return 10 * 10;
}

tenSquared();

function nineSquared() {
  return 9 * 9;
}

nineSquare();

tenSquared and nineSquared are nearly identical functions, only changing the number. Here we are breaking 🔴 DRY - Don’t Repeat Yourself principle.

We can make the generalized function to make it reusable by accepting a parameter:

• Generalizing Functions

Parameters (placeholders) means we don't need to decide what data to run our functionality or until we run the function.

• Then provide an actual value (argument) when we run the function.

Follows the same principle, we may not want to decide exactly what some of our functionality is until we run our function. Higher Order Functions are those function which takes function as an argument or return a function.

• Higher Order Functions

Callbacks & Higher Order Functions simplify our code and keep it DRY, Callbacks are core aspect of async JavaScript, and are under-the-hood of promises, async/await.

Summary

Functions in JavaScript are first class objects, They can be treated like any other JavaScript object

1. Assigned to variables and properties of other objects.
2. Passed as arguments into functions.
3. Returned as values from functions.

Higher Order Functions & Callbacks are crucial in JavaScript because

- Enable Functional Programming
- Eliminates DRY principle (map, filter and reduce)
- Enable asynchronouse programming.