A function can be thought of as a machine that performs a specific task. Such as make sandwich. Every time you want a sandwich to be made, you just have to turn on the machine. Or execute the function, as programmers would say.

Functions are important in programming because it allows programmers to dissect parts of the program where the same task is carried out repeatedly.

Instead of repeating all the instructions to make a sandwich. The programmer can pack all the instructions inside a function, call it makeSandwich. And execute the function whenever a sandwich needs to be made.

The following is a simple function written in JavaScript. It will add the two input numbers and return their sum.

var sum = function (a, b) {
  return a + b;
}
// sum(3, 7) will return 10

Functions are important as it helps keep our programs DRY (Don't Repeat Yourself). Whenever you find yourself repeating identical or similar code, you should consider abstracting it into a function.

Another neat thing about functions is that they can call themselves, these are called recursive functions. Programmers often rejoice whenever they see an elegant recursive function in use.