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README.md |
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 thearguments
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 theArrowFunctionExpression
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.