More on functions

Last week, we started our discussion on JavaScript functions. If you need a recap on functions, these two videos will be helpful.

In this lesson, we dive deeper into more details about functions.

Scope of variables in Functions

Variables defined within the scope of a function has a lifespan within that specific function alone and not available outside of the function in which it was defined. Step through the code snippet and watch the demonstration to see how the scopes of the functions and variables change.

1 function first(val){
2	  var name = val
3	  console.log(name)
4	  
5	  var addr = "residence"
6	  
7	  function second(val){
8	    var name = val
9	    console.log(name)
10	  }
11	  
12	  second(addr)
13 }

Arrow Functions

We talked a little bit about arrow functions last week, now let's see an example. Instead of the function keyword, we use an arrow => (the equal sign, followed by the greater than symbol). The arrow comes after the list of parameters, and is followed by the function body.

let power = (base, exponent) => {
    return base ** exponent;
}

Note that if you have a single parameter, you can ignore the parenthesis in your arrow function. If the function has no parameter, simply put in an empty parenthesis. If the function is a single expression, you can also ignore the curly brackets and the return statement.

let power = (base, exponent) =>  base ** exponent;

console.log(power(4,3));

Higher-order Functions

Higher-order functions are functions that operate on other functions, either by taking a function as an argument or by returning a function. In JavaScript, there are a number of built-in higher order functions, that take in functions as arguments.

We saw the map() function in the previous lesson. The map function takes in a function. You might also have a need to build your own higher-order function.

Let's look at an example of a function that is passed as a parameter to another function.