Javascript 变量相关基础

来自JSDG：

引用

The types can be divided into two groups: primitive types and reference types.

Numbers, boolean values, and the null and undefined types are primitive.

Objects, arrays, and functions are reference types.

A primitive type has a fixed size in memory.

Variables hold the actual values of primitive types, but they hold only references to the values of reference types.

The JavaScript keyword null is a special value that indicates no value. null is usually considered a special value of object typea value that represents no object.


undefined is returned when you use either a variable that has been declared but never had a value assigned to it or an object property that does not exist.

null and the undefined value are distinct, the == equality operator considers them equal to one another.

When the undefined value is used in a Boolean context, it converts to false.
When used in a numeric context, it converts to NaN.
when used in a string context, it converts to "undefined".

变量分为两个类型，原始类型和引用类型。

原始类型：数字、布尔、null、undefined
引用类型：对象、数组、函数

字符串是特例，拥有原始类型和引用类型的特点

一个变量持有原始类型的实际值，但是只持有引用类型的引用：

var a = [1,2,3];  // Initialize a variable to refer to an array
var b = a;        // Copy that reference into a new variable
a[0] = 99;        // Modify the array using the original reference
alert(b);         // Display the changed array [99,2,3] using the new reference

null是特殊的类型，对象类型的数据默认值就是null，代表“没有对象”。

undefined意味着这个变量要么没有声明，要么就是指向一个并不存在的对象属性

JS中null==undefined的值是true，要用===才能区别这两个特殊值


有相关自动转换的情况：


在函数内部申明的局部变量一定要用var，否则就会默认得到一个全局变量