Objects

Question 1

Empty Object

Answer 1

An empty object (“empty cabinet”) can be created using one of two syntaxes:
let user = new Object(); // “object constructor” syntax
let user = {}; // “object literal” syntax

Question 2

Key Value pairs

Answer 2

Colonised Key Value pairs Comma Separated
let user = { // an object
name: “John”, // by key “name” store value “John”
age: 30 // by key “age” store value 30
};

alert( user.name ); // John
Values can be of any Javascript primitive type like boolean, string integer etc
To remove a property, we can use the delete operator:
delete user.age;

Question 3

Property exists test

Answer 3

let user = { name: “John”, age: 30 };

alert( “age” in user ); // true, user.age exists

Question 4

for…in loop

Answer 4

for (key in object) {
// executes the body for each key among object properties
}

Question 5

Object method this

Answer 5

let user = {
name: “John”,
age: 30,

sayHi() {
// “this” is the “current object”
alert(this.name);
}

};

user.sayHi(); // John

Question 6

this is not bound

Answer 6

In JavaScript, keyword this behaves unlike most other programming languages. It can be used in any function, even if it’s not a method of an object.
function sayHi() {
alert( this.name );
}
The value of this is evaluated during the run-time, depending on the context.

let user = { name: “John” };
let admin = { name: “Admin” };

function sayHi() {
alert( this.name );
}

// use the same function in two objects
user.f = sayHi;
admin.f = sayHi;

// these calls have different this
// “this” inside the function is the object “before the dot”
user.f(); // John (this == user)
admin.f(); // Admin (this == admin)

Question 7

this undefined

Answer 7

function sayHi() {
alert(this);
}

sayHi(); // undefined

Question 8

Arrows and this

Answer 8

Arrow functions are special: they don’t have their “own” this. If we reference this from such a function, it’s taken from the outer “normal” function.

let user = {
firstName: “Ilya”,
sayHi() {
let arrow = () => alert(this.firstName);
arrow();
}
};

user.sayHi(); // Ilya

Question 9

Constructor Function

Answer 9

Constructor functions technically are regular functions. There are two conventions though:

They are named with capital letter first.
They should be executed only with “new” operator.

function User(name) {
this.name = name;
this.isAdmin = false;
}

let user = new User(“Jack”);

Question 10

Constructor Return Type

Answer 10

Usually, constructors do not have a return statement. Their task is to write all necessary stuff into this, and it automatically becomes the result.
You can override it to return an object though, but that may not be a good practice.