First Class Functions

In simple terms, First Class functions just means that functions are normal variables. They can do anything a normal variable can do, such as be added to objects, put in arrays or passed as parameters. The only thing special about them is that they can be invoked.

Syntax

The most common form of function as a variable these days is array functions, but they can be any sort of function.

// Function Syntax
function add(a, b) {
    return a + b;
}

// NOTE: We don't actually need to assign
//       add to a variable like we do here.
//       It is already one!
//       I have just assigned it here to
//       make it clear what is happening.
const addAsVariable = add;

// Arrow Function Syntax
const arrowAdd = (a, b) => a + b;

// These are both valid :)
addAsVariable(1,2); // 3
arrowAdd(1,2); // 3

// Function Syntax
function add(a: number, b: number) {
    return a + b;
}

const addAsVariable = add;

// Arrow Function Syntax
const arrowAdd = (a: number, b: number) => a + b;

// These are both valid :)
addAsVariable(1,2); // 3
arrowAdd(1,2); // 3

Why is this useful?

Now that we know they are just like any other variable, we can start passing functions around and extending their functionality. What we can do is keep our functions really small, then build them up into bigger and better things as we go.

Passing as parameters

One of the best things about First Class functions is being able to pass through code to Pure functions, which lets you have dynamic code in something pure.

It is also really common in javascript to pass through a callback to an asynchronous function that will be run when required. This was more common before promises, but you will still see it about.

// Check our internet connection
async function ping() {
    const response = await fetch('www.google.com');
    if (!response.ok) {
        alert("Connection Lost!");
    }
}

// Here we pass our function as a parameter
// just like a normal variable
setInterval(ping, 60000);

There is not really anything more to them. Just treat them like a normal variable and you will have First Class functions.

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