function

* function needs to be defined first and then it can be called(executed)
```js
// definition
function add(a, b) {
  return a + b;
}

// execution or you can say call function add with two parameters 1 and 2
add(1, 2);
```

* function body
All lines of code between `{` and `}` is called `function body`.
```js
function add(a, b) {
  ... // more code
  ...
  ...
  ...
  ...
  return a + b;
}
```


* parameters and arguments
```js
// a and b called arguments like placeholders
// in this example, a holds value 1, b holds value 2
// to access value 1, we use a - console.log(a) will get u 1
// to access value 2, we use b - console.log(b) will get u 2
function add(a, b) {
  return a + b;
}

add(1, 2); // 1 and 2 called parameters - THE ACTUAL VALUES!!!
```

* return value
Functions can have return values.

```js
function add(a, b) {
  return a + b; // return the value to outside
}

var res = add(1, 2);
console.log(res); // 3
```

Functions can have no return values sometimes.

```js
function add(a, b) {
  return; // return nothing!
}

var res = add(1, 2);
console.log(res); // undefined
```

By default, function returns `undefined`.

```js
function add(a, b) {
  a + b;
}

var res = add(1, 2);
console.log(res); // undefined because u don't return anything in function add
```

* Code after return statement
Code after return statement will not be executed!

```js
function add(a, b) {
  var res = a + b;
  return res;
  // anything below will not run!!!!!!!!
  var b = 155;
  return b;
}

var res = add(1, 2);

console.log(res); // 3 not 155
```