Function Definition

function printKing() {
    console.log('avi biter');
}

printKing();

Function Value

just like other values:

const nisim = 42;
const shlomo = 'gimel yafit';
const david = false;

you can assign a function value to a variable

const printSinger = function() {
    return 'shimi tavori';
};

We can this function with the variable just like a function defined regularly:

const printKing = function() {
    console.log('avi biter');
};

printKing();

Warning

assignment of function value is not hoisted as regular function definition.

printKing(); // works fine

function printKing() {
    console.log('avi biter');
}

printKing(); // error!
// Uncaught ReferenceError: can't access lexical
// declaration 'printKing' before initialization

const printKing = function() {
    console.log('avi biter');
};

Callback

a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

const greeting = function(name) {
	alert('Hello ' + name);
};

function processUserInput(callback) {
	const name = prompt("Please enter your name.");
	callback(name);
}

processUserInput(greeting);

Use Alias

instead of definning the function and giving it a name we can pass it’s definition as an argument to the processUserInput function.

function processUserInput(callback) {
	const name = prompt("Please enter your name.");
	callback(name);
}

processUserInput(function(name) {
	alert('Hello ' + name);
});

Arrow Function

Multiple Lines + Multiple Parameters

const doSomething = function(p1, p2, p3) {
    // first line
    // second line
    // ...
};

arrow version

const doSomething = (p1, p2, p3) => {
    // first line
    // second line
    // ...
};

Multiple Lines + Single Parameter

const doSomething = function(p1) {
    // first line
    // second line
    // ...
};

arrow version

const doSomething = (p1) => {
    // first line
    // second line
    // ...
};

with a single parameter the parenthesis can be omitted

const doSomething = p1 => {
    // first line
    // second line
    // ...
};

Single Return Line + Multiple Parameters

const getSomething = function(p1, p2, p3) {
    return 'buldozer';
};

arrow version

const getSomething = (p1, p2, p3) => {
    return 'buldozer';
};

shorthand: with a single return line the block parenthesis and the return word can be omitted.

const getSomething = (p1, p2, p3) => 'buldozer';

Single Return Line + Single Parameter

const getDouble = function(num) {
    return 2 * num;
};

arrow version

const getDouble = (num) => {
    return 2 * num;
};

shorthand: as explained before

const getDouble = num => 2 * num;