Named function (Function declaration)

function helloWorld() {
   console.log('Hello World');
}

helloWorld();

Named functions are used when the codes need to be executed several times.

Anonymous function

let helloWorld = function () {
   console.log('Hello World');
}

console.log(helloWorld);

Anonymous functions are used the codes needs to be executed only once or twice and also used in event handler as below:

.onclick = function() {
   console.log('Hello World')
}

Arrow function

const helloWorld = () => 'Hello World';

console.log(helloWorld);

Arrow functions are shorter ways of function expressions from ES6 and useful to write functions in short ways.