Anonymous Functions vs Named Functions vs Arrow Functions

First you learn the syntax to create a function, and that's fine. Then you start hearing about anonymous functions, and they look a bit different and you're not entirely sure why the world needs them. Then arrow functions rear their arrow-y head and you're thoroughly confused...at least, I was. For me to better understand the differences of all three, I needed to put them side by side and compare them.

This is a named function, aka a function declaration

function brag(count) {
     return("I can do " + count + " pushups");
} 

console.log(brag(3)) // I can do 3 pushups

This is an anonymous function, aka a function expression

var brag = function(count) {
     return("I can do " + count + " pushups");
} 

console.log(brag(3)) // I can do 3 pushups

This is an arrow function, a special type of function expression.

var brag = (count) => ("I can do " + count + " pushups");

console.log(brag(3)) // I can do 3 pushups

It's still a function expression because everything to the right of the = is a value that we assign to the variable brag.

Note that (1) there's no curly braces used to define the code block, and (2) there's also no return statement. That's because an arrow function has implicit return, meaning the value is returned automatically. You don't actually need any parentheses either, but I left them in because they make the code more readable.

Besides the syntax, how are they different?

Function declarations are hoisted, which means they are loaded into memory at compilation. That's why in the example below, the function call works even before the function declaration appears.

console.log(brag(3)) // I can do 3 pushups

function brag(count) {
     return("I can do " + count + " pushups");
} 

console.log(brag(3)) // I can do 3 pushups

Anonymous functions, on the other hand, are not hoisted. As you can see, when we call the "brag" function before the function declaration, we get an error. When we call it after the declaration, it works.

console.log(brag(3)) // TypeError: brag is not a function

var brag = function(count) {
     return("I can do " + count + " pushups");
} 

console.log(brag(3)) // I can do 3 pushups

Why would you use an anonymous function instead of a named function?

Sometimes you don't need to name a function because you're just going to use it as a callback to another function. Since you're not using it again elsewhere, it doesn't need a name.

For example, here we're using a named function called 'brag' (also known as a function declaration):

function brag(count) {
     return("I can do " + count + " pushups");
} 

console.log(brag(3)) // I can do 3 pushups

...but we could just as well make it anonymous, like so:

console.log(function(count) {
     return("I can do " + count + " pushups");
} (3)) // I can do 3 pushups

Why would you use an arrow function instead of an anonymous function?

I used to think that Arrow functions were just shorter alternatives to anonymous expressions. But Ben Calder correctly pointed out that "Arrow functions preserve the context of this, which solves some really confusing scoping issues that previously required closures or bind(this). There's some discussion of this in the MDN docs." In summary, when you use this inside an arrow function, it will always keep it's context and not change it to something unexpected at runtime.

In the comments, Angela mentions that you can read "additional information on how the arrow function is different here". Thank you Ben and Angela for nudging me to update this post!