JAVASCRIPT

#################################

COMMENTS

// This is a single line comment

/*
 And this is
 a multi-line
 comment
 */
 
 
 
 
 
 
 
 
 
 
#################################
 
VARIABLES
 
// This is how you define a variable
// `x` will be `undefined`
var x;

// Declare a constant (the convention is to use CAPS for constants)
const FOO = 42;

// Declare another two variables, using `var` and `let`
var hello = 'world';
let bar = 'baz';

let foo = 42;
foo = 'bar';
foo
→ 'bar'

 
 
 
 
 
 
 
 
 
 
#################################

IF STATEMENTS
 
let foo = 42;

if (foo > 40) {
    // do something
} else {
    // do something else
}
 
 
 
 
 
 
 
 
 
 
 
#################################
 
SWITCH STATEMENTS
 
let planet = 'Earth';

switch (planet) {
  case "Mercury":
  case "Venus":
    console.log("Too hot here.");

  case 'Earth' :
    console.log("Welcome home!");
    break;

  case 'Mars' :
    console.log("Welcome to the other home!");
    break;

  case "Jupiter":
  case "Saturn":
  case "Uranus":
  case "Neptune":
  case "Pluto":
    console.log("You may get gold here.");
    break;

  default:
    console.log("Seems you found another planet.");
    break;
}










#################################
 
OBJECTS
 
let myObj = { world: "Earth" };
 
 let firstObj = {};
let secondObj = {};

// Check if they are equal
firstObj === secondObj
→ false

// Comparing an object with itself...
firstObj === firstObj
→ true

// Let's point the secondObj to firstObj
secondObj = firstObj

// Now they are equal
firstObj === secondObj
→ true
 
 
 
 
 
 
 
 
 
 
#################################

ARRAYS

let fruits = ["apples", "pears", "oranges"];









#################################

ACCESS, ADD, REMOVE, AND UPDATE ELEMENTS

fruits[1]
→ "pears"

> let fruits = ["Apple"]

// Add to the end of the array
> fruits.push("Pear")
2 // < The new length of the array

 [ 'Apple', 'Pear' ]
 //         ^ This was just added

// Add to the start of the array
> fruits.unshift("Orange")
3 // < The new length of the array

 [ 'Orange', 'Apple', 'Pear' ]
 // ^ This was just added

// Access the element at index 1 (the second element)
> fruits[1]
'Apple'

// How many items do we have?
> fruits.length
3

// Turn the Apple into Lemon
> fruits[1] = "Lemon"
'Lemon'

 [ 'Orange', 'Lemon', 'Pear' ]
 //          ^ This was before an Apple

// Insert at index 1 a new element
> fruits.splice(1, 0, "Grapes")
[] // < Splice is supposed to delete things (see below)
   //   In this case we're deleting 0 elements and
   //   inserting one.

 [ 'Orange', 'Grapes', 'Lemon', 'Pear' ]
 //          ^ This was added.

// Get the Lemon's index
> fruits.indexOf("Lemon")
2

// Delete the Lemon
> fruits.splice(2, 1)
[ 'Lemon' ]

 [ 'Orange', 'Grapes', 'Pear' ]
 //                   ^ Lemon is gone

// Remove the last element
> fruits.pop()
'Pear'

 [ 'Orange', 'Grapes' ]
 //                  ^ Pear is gone

// Remove the first element
> fruits.shift()
'Orange'

   [ 'Grapes' ]
 // ^ Orange is gone











#################################

ITERATING OVER ARRAYS

// Using for-loop
var arr = [42, 7, -1]
for (var index = 0; index < arr.length; ++index) {
    var current = arr[index];
    /* Do something with `current` */
}

// Others prefer defining an additional `length` variable:
for (var index = 0, length = arr.length; index < length; ++index) {
    var current = arr[index];
    /* ... */
}

// Another way i using `forEach`:
arr.forEach((current, index, inputArray) => {
    /* Do something with `current` */
});

// Or using the for ... of:
for (let current of arr) {
    /* current will be the current element */
}










#################################

FUNCTIONS

function sum (a, b) {
    return a + b;
}

var sum = function (a, b) {
    return a + b;
}

// Using the ES6 arrow functions
let sum = (a, b) => a + b;









#################################

CONSTRUCTORS AND CLASSES

class Person {
  constructor (name) {
    this.name = name;
  }
  getName () {
    return this.name;
  }
}

var p = new Person("Carol");
console.log(p.getName());
// => "Bob"



class Musician extends Person {
   constructor (name, instrument) {
     // Call the base class
     super(name);

     this.instrument = instrument;
   }

   play () {
     console.log(`${this.getName()} is playing ${this.instrument}`);
   }
}

var me = new Musician("Johnny B.", "piano");

// Get the name of the musician, who is also a person
console.log(me.getName());
// => "Johnny B."

me.play();
// => "Johnny B. is playing piano."









#################################

CREATE AND THROW ERRORS

let myError = new Error("Something went really wrong.");

// You can even append custom data here
myError.code = "MY_FANCY_ERROR_CODE";

throw new Error("Something bad happened.");

function sayHello(message) {
  if (typeof message !== "string") {
    throw new TypeError("The message should be a string.");
  }
  console.log(message);
}









#################################

LOOPS

for (var i = 1; i <= 42; ++i) {
  console.log(i);
}

var name = {
  first: "Johnny",
  last: "B."
};

for (var key in name) {
  if (name.hasOwnProperty(key)) {
    console.log(key, name[key]);
    // "first", "Johnny"
    // "last", "B."
  }
}

let numbers = [-1, 7, 42, 64];
for (let num of numbers) {
  console.log(num);
}
// -1
// 7
// 42
// 64

var i = 1;
while (i <= 42) {
    console.log(i);
    ++i;
}

var i = 0;
do {
    ++i;
    console.log(i);
} while (i < 42);










#################################

REGULAR EXPRESSIONS

/^[0-9]+$/gm.test("a")
// => false

/^[0-9]+$/gm.test("42")
// => true






#################################

Dank aan Johnny B. (https://www.codementor.io/johnnyb/javascript-cheatsheet-fb54lz08k)