JavaScript 对象
In JavaScript, objects are king. If you understand objects, you understand JavaScript.
In JavaScript, almost "everything" is an object.
- Booleans can be objects (if defined with the
newkeyword) - Numbers can be objects (if defined with the
newkeyword) - Strings can be objects (if defined with the
newkeyword) - Dates are always objects
- Maths are always objects
- Regular expressions are always objects
- Arrays are always objects
- Functions are always objects
- Objects are always objects
All JavaScript values, except primitives, are objects.
JavaScript Primitives
A primitive value is a value that has no properties or methods.
3.14 is a primitive value
A primitive data type is data that has a primitive value.
JavaScript defines 7 types of primitive data types:
Examples
stringnumberbooleannullundefinedsymbolbigint
Immutable
Primitive values are immutable (they are hardcoded and cannot be changed).
if x = 3.14, you can change the value of x, but you cannot change the value of 3.14.
| Value | Type | Comment |
|---|---|---|
| "Hello" | string | "Hello" is always "Hello" |
| 3.14 | number | 3.14 is always 3.14 |
| true | boolean | true is always true |
| false | boolean | false is always false |
| null | null (object) | null is always null |
| undefined | undefined | undefined is always undefined |
Objects are Variables
JavaScript variables can contain single values:
JavaScript variables can also contain many values.
对象也是变量。但对象可以包含许多值。
Object values are written as name : value 对（名称和值用冒号分隔）。
A JavaScript object is a collection of named values
使用以下方式声明对象是一种常见的做法 const 关键词。
对象属性
The named values, in JavaScript objects, are called 特性.
| 财产 | Value |
|---|---|
| 名 | 约翰 |
| 姓 | 多伊 |
| 年龄 | 50 |
| 眼睛的颜色 | 蓝色的 |
Objects written as name value pairs are similar to:
- Associative arrays in PHP
- Dictionaries in Python
- Hash tables in C
- Hash maps in Java
- Hashes in Ruby and Perl
对象方法
方法包括 行动 可以对对象执行的操作。
Object properties can be both primitive values, other objects, and functions.
An object method is an object property containing a 函数定义.
| 财产 | Value |
|---|---|
| 名 | 约翰 |
| 姓 | 多伊 |
| 年龄 | 50 |
| 眼睛的颜色 | 蓝色的 |
| 全名 | function() {return this.firstName + " " + this.lastName;} |
JavaScript objects are containers for named values, called properties and methods.
You will learn more about methods in the next chapters.
Creating a JavaScript Object
With JavaScript, you can define and create your own objects.
There are different ways to create new objects:
- Create a single object, using an object literal.
- Create a single object, with the keyword
new. - Define an object constructor, and then create objects of the constructed type.
- Create an object using
Object.create().
Using an Object Literal
This is the easiest way to create a JavaScript Object.
Using an object literal, you both define and create an object in one statement.
An object literal is a list of name:value pairs (like age:50) inside curly braces {}.
The following example creates a new JavaScript object with four properties:
空格和换行并不重要。对象定义可以跨越多行：
This example creates an empty JavaScript object, and then adds 4 properties:
例子
const person = {};
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
亲自尝试 »
Using the JavaScript Keyword new
The following example create a new JavaScript object using new Object(), and then adds 4 properties:
例子
const person = new Object();
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
亲自尝试 »
The examples above do exactly the same.
But there is no need to use new Object().
For readability, simplicity and execution speed, use the object literal method.
JavaScript Objects are Mutable
Objects are mutable: They are addressed by reference, not by value.
If person is an object, the following statement will not create a copy of person:
const x = person; // Will not create a copy of person.
The object x is not a copy of person. It is person. Both x and person are the same object.
Any changes to x will also change person, because x and person are the same object.
例子
const 人 = {
firstName:"John",
lastName:"Doe",
age:50, eyeColor:"blue"
}
const x = person;
x.age = 10; // Will change both x.age and person.age
亲自尝试 »
