Objects ​

In JavaScript, an object is an unordered collection data type that consists of key-value pairs. Objects are used to describe real-world entities. For example, to describe a mischievous child named "Xiao Ming," we can define an object with several properties:

let xiaoming = {
    name: '小明',
    birth: 1990,
    school: 'No.1 Middle School',
    height: 1.70,
    weight: 65,
    score: null
};

In JavaScript, an object is represented with {...}, and key-value pairs are defined in the format key: value, separated by commas. Note that the last key-value pair should not have a trailing comma, as it may cause errors in some browsers (like older versions of IE).

Accessing Properties ​

You can access properties using the dot (.) operator, which requires that the property name is a valid variable name:

xiaoming.name; // '小明'
xiaoming.birth; // 1990

If a property name contains special characters or hyphens, you must enclose it in quotes and use bracket notation:

let xiaohong = {
    name: '小红',
    'middle-school': 'No.1 Middle School'
};
xiaohong['middle-school']; // 'No.1 Middle School'
xiaohong['name']; // '小红'

While you can access properties with both dot and bracket notation, using dot notation is simpler for standard variable names.

Dynamic Nature ​

JavaScript objects are dynamic, allowing you to freely add or delete properties:

let xiaoming = {
    name: '小明'
};
xiaoming.age; // undefined
xiaoming.age = 18; // Adds the age property
delete xiaoming.age; // Deletes the age property

If you access a non-existent property, JavaScript returns undefined without throwing an error:

'use strict';
let xiaoming = {
    name: '小明'
};
console.log(xiaoming.name); // '小明'
console.log(xiaoming.age); // undefined

Checking for Properties ​

To check if an object has a certain property, you can use the in operator:

'name' in xiaoming; // true
'grade' in xiaoming; // false

Be cautious, as in checks both own properties and inherited properties:

'toString' in xiaoming; // true

To determine if a property is directly on the object (not inherited), use hasOwnProperty():

xiaoming.hasOwnProperty('name'); // true
xiaoming.hasOwnProperty('toString'); // false

Summary ​

JavaScript objects are versatile data structures that allow for the dynamic management of properties, providing a powerful way to represent real-world entities in code.