let user = new Object(); // “object constructor” syntax let user = {}; // “object literal” syntax

let user = { // an object name: “John”, // by key “name” store value “John” age: 30 // by key “age” store value 30 };

A property has a key (also known as “name” or “identifier”) before the colon ":" and a value to the right of it.

alert( user.name ); // John alert( user.age ); // 30

we can also add values like user.place = “delhi”

and also delete delete user.age;

can use multiword properties e.g. let user = { name: “John”, age: 30, “likes birds”: true, // multiword property name must be quoted }; user[likes bird]

end last property with hanging comma to make it easy to remove add new alement

use square bracker [ ] to give a value of variable to a key for eg, let var = “age” let user = { [var]:9 } sq. bracket can also be used to call values like

user[age] we cannot use user.var but can use user[var]

this also called computed properties:

let fruit = ‘apple’; let bag = { [fruit + ‘Computers’]: 5 // bag.appleComputers = 5 };

**object can also have functions called methods like

const person = {
  name: ["Bob", "Smith"],
  age: 32,
  bio: function () {
    console.log(`${this.name[0]} ${this.name[1]} is ${this.age} years old.`);
  },
  introduceSelf: function () {
    console.log(`Hi! I'm ${this.name[0]}.`);
  },
};

Constructors

used to create object like :

function createPerson(name) {
  const obj = {};
  obj.name = name;
  obj.introduceSelf = function () {
    console.log(`Hi! I'm ${this.name}.`);
  };
  return obj;
}

property value shorthand

function makeUser(name, age) { return { name: name, age: age, // …other properties }; }

let user = makeUser(“John”, 30); alert(user.name); // John

can also be written as

function makeUser(name, age) { return { name, // same as name: name age, // same as age: age // … }; }

can mix normal properties and shorthand let user = { name, // same as name:name age: 30 };

property name limits

As we already know, a variable cannot have a name equal to one of the language-reserved words like “for”, “let”, “return” etc.

But for an object property, there’s no such restriction:

___proto__

let obj = {};
obj.__proto__ = 5; // assign a number
alert(obj.__proto__); // [object Object] - the value is an object, didn't work as intended

as this is because proto is used for prototypal inheritence

in

let user = { age: 30 };

let key = “age”; alert( key in user ); // true, property “age” exists

why not this : let user = {};

alert( user.noSuchProperty === undefined ); // true means “no such property”

because this doesnt work for

let obj = { test: undefined };

alert( obj.test ); // it’s undefined, so - no such property?

alert( “test” in obj ); // true, the property does exist!

for in

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

for (let key in user) {
  // keys
  alert( key );  // name, age, isAdmin
  // values for the keys
  alert( user[key] ); // John, 30, true
}

order?

integer properties are ordered e.g.

  "49": "Germany",
  "41": "Switzerland",
  "44": "Great Britain",
  // ..,
  "1": "USA"
};
 
for (let code in codes) {
  alert(code); // 1, 41, 44, 49
}

but strings are not ordered; to unorder the integer keys we could use + for eg “+91” : “ok”

integer property wo hoti jo agar integer m transform kro fir wapas string m toh same rehti “49” h “1.2” ni h “+49” ni h kyuki usko int m krne se wo 49 hojata + chala jata

SUMMARY

Objects are associative arrays with several special features.

They store properties (key-value pairs), where:

Property keys must be strings or symbols (usually strings). Values can be of any type. To access a property, we can use:

The dot notation: obj.property. Square brackets notation obj[“property”]. Square brackets allow taking the key from a variable, like obj[varWithKey]. Additional operators:

To delete a property: delete obj.prop. To check if a property with the given key exists: “key” in obj. To iterate over an object: for (let key in obj) loop. What we’ve studied in this chapter is called a “plain object”, or just Object.

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.

Quartz 4

Explorer

Object