public/subjects/what-is-my-purpose/README.md

What is my purpose ?

You have seen how to call functions that were stored in object properties.

Remember this example of function call ?

//       ↙ identifier, like variables
console.log('Hello There !') // <- function call happening here
//          ↖ open paren + argument + close paren

or these ones:

let roundedValue = Math.round(1.8) // another function call happening here
console.log(roundedValue) // and another one here

There, we saw how to call and use "built-in" functions.

Here, now, we are going to learn how to declare our owns. This will gives us even more freedom to build our own logic.

Declaring a function

We mentioned before that a function had to be either assigned to a variable or an object property.

We will now declare one in a variable. Let's call it: myFirstFunction.

Once a variable is declared; Remember how an array can be recognized with this syntax : [] ? or an object with this one : {} ?

Well, we will use another syntax for declaring our function. We will do so using the ArrowFunctionExpression syntax : () => {}

• So we first put parens (), These, are the containers of the arguments that go in the function. For now we will leave them empty with no arguments. (More on those later on)
• We then add the arrow => which is the distinguishing feature of the ArrowFunctionExpression syntax.
• Finally, we add the curly brackets {} to delimite the scope of our newly created function. Note: They are not always necessary, you will probably find examples of this function syntax without the {}. However, for now because you are learning. We will put them most of the time.

//    ↙ normal variable     ↙ beginning of the scope of the function
let myFirstFunction = () => {
  //                    ↖ parens () for arguments and the arrow => for syntax
} // <-end of the scope of the function

It's now possible to call this function using the (), like any pre-declared function:

myFirstFunction() // nothing happens

This function if called, does not do anything, since it doesn't contain any code.

The scope of a function {}

Very much like an if statement a function has a scope. The scope in between the curly braces{} is where the action happens. Let's add something to the scope of our function.

let myFirstFunction = () => {
  console.log('Calling my first function')
  // ↖ some instructions to do when the function is called !
}

Now the function if called, display the output the console.log().

myFirstFunction() // "Calling my first function"

We actually used a function and gave it this single instruction:

• call another function console.log().

Instructions

You are a robot made by a scientist called Rick and you want to know your purpose.

• Declare a function named ask that log 'What is my purpose ?' in the console
• Declare a function named reply that log 'You pass butter.' in the console

Then first call the ask then the reply once, in that order.