How to Create Functions in JavaScript

In a real-world project, you may have pieces of code for different purposes.

For instance, you could have a piece of code asking the user to provide an input, another piece of code processing the user request, and so on.

If not organized appropriately, future maintenance would become a nightmare.

Not only because it is very easy to lose track, but also because there is no way to reuse your code.

Luckily, there is a fix, functions.

Function is a way for you to organize and reuse pieces of your code. It can be created with the function keyword, as demonstrated below:

function countEven(arr) {
  let n = 0;
 
  for (let e of arr) {
    if (e % 2 === 0) {
      n++;
    }
  }
 
  return n;
}

This function takes an input:

function countEven(arr) {
  let n = 0;
 
  for (let e of arr) {
    if (e % 2 === 0) {
      n++;
    }
  }
 
  return n;
}

Does something with the input:

function countEven(arr) {
  let n = 0;
 
  for (let e of arr) {
    if (e % 2 === 0) {
      n++;
    }
  }
 
  return n;
}

And finally return a result:

function countEven(arr) {
  let n = 0;
 
  for (let e of arr) {
    if (e % 2 === 0) {
      n++;
    }
  }
 
  return n;
}

To call the function, use its name, and followed by a pair of parenthesis where you can provide the function input.

function countEven(arr) {
  let n = 0;
 
  for (let e of arr) {
    if (e % 2 === 0) {
      n++;
    }
  }
 
  return n;
}
 
let arr = [1, 2, 3];
 
console.log(countEven(arr)); // -> 1