Objects are associative arrays with several special features.

Property existence test, “in” operator

There’s also a special operator "in"

"key" in object

let user = { name: "John", age: 30 };

alert( "age" in user ); // true, user.age exists
alert( "blabla" in user ); // false, user.blabla doesn't exist

Order of for in loop

let codes = {
  "49": "Germany",
  "41": "Switzerland",
  "44": "Great Britain",
  // ..,
  "1": "USA"
};

for (let code in codes) {
  alert(code); // 1, 41, 44, 49
} **// Works only with number (sorted)**

in below example , this will print how it should

let codes = {
  "+49": "Germany",
  "+41": "Switzerland",
  "+44": "Great Britain",
  // ..,
  "+1": "USA"
};

for (let code in codes) {
  alert( +code ); // 49, 41, 44, 1  non-numbers
}

There are many other kinds of objects in JavaScript:

• Array to store ordered data collections,
• Date to store the information about the date and time,
• Error to store the information about an error.
• …And so on.

Garbage Collection

this

Constructor function