Function 
function square(number) {
  return number * number;
} 

square(2); // 4

A simple javascript function codeblock to execute code. Functions have their own local scope and are hoisted, accessible anywhere once defined. Exit out once they hit a return statement.

Functions JS
Recursion 
function pow(x, n) {
  if (n == 1) {
    return x;
  } else {
    return x * pow(x, n - 1);
  }
}

alert( pow(2, 3) ); // returns to the power of 3 = 8

When a function calls itself.

Functions JS
Function Expressions 
const func = function(param){
  // code here
}

func();

Functions can be assigned to a variable. These functions are anonymous functions. Function expressions are not hoisted and only work after where defined.

Functions JS
Function Parameters 
function doubleIt(a, b = 2) {
  return a * b;
}

console.log(doubleIt(5)); // 10

Parameters take in values to be used in functions. Can also have predefined default parameters.

Functions JS
Destructuring Functions 
let truck = {
  model: 'Challenger',
  maker: 'Dodge',
  city: 'Chicago',
  year: '2014',
  convertible: false
};

const printCarInfo = ({model, maker, city}) => {
  console.log(`The ${model}, or ${maker}, is in the city ${city}.`); 
};

printCarInfo(truck);
// Prints: The 2014 Challenger, or Dodge, is in the city Chicago.

Functions can use destructuring assignment by using its parameters.

Functions JS
Callback Functions 
function callbackFunction(){
  console.log('Hello');
};

function executeCallback(callback){
  callback()
};

executeCallback(callbackFunction);

A callback function is a function passed into another function as an argument.

Functions JS
Arrow Functions 
const doubleIt = (a, b = 2) => a * b;

console.log(doubleIt(5));

Arrow functions provide shorter syntax. Don't require function keyword, return, if returning one line of code, and parentheses, if using only one parameter.

Functions JS
Return 
function rollDice() {
return Math.floor(Math.random() * 6) + 1;
}

const dieRoll = rollDice();

If encountered, jumps out of the function and returns given value. Can store and use these values.

Functions JS
IIFE 
Arrow function syntax:
(() => {
  // …
})();

Async await syntax:
(async () => {
  // …
})();

IIFE's are functions that are immediately invoked function expressions. Runs as soon as it is defined, helps avoid polluting the global scope.

Functions JS