Introduction to js basic syntax: data types and variable types

This article brings you an introduction to the basic syntax of js: data types and variable types, which have certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1. Unchangeable original values ​​(stack data) (five)

Number (number), string (string), Boolean value (boolean), undefined, null

Among them; undefined means undefined, and null means empty. The difference between them is that null has a value, but this value is an empty value, while undefined means undefined and has no value at all. null is generally used as a placeholder.

var a = 4;
var b = a;
a = 10;
console.log(b);//4//原始值是把一个值a放到另一个值b里面,改了第一个值a,第二个之值b不变

2. Reference value (heap data)

Array, object, function

Note: As long as a variable is assigned a value, the memory will be re-applied

var arr = [1, 2];
var arr1=arr;
arr.push(3);
console.log(arr1); //[1, 2, 3]
//引用值时把第一个值放到第二个值里面,改变第一个值,第二个值也改变
//引用值是在栈内存里面存放堆的地址,拷贝的是地址,所以改变了arr,实际上是改变了arr指向的地址内存放的内容,而arr和arr1指向同一个地址,故arr1也跟着变

Address

Variable

Stored value

0x0001

arr

[1,2] [1,2,3] e7dea7ebec345b95908f3946bdf704ce{m:1}

　　 b: {0x0001}--->{n:0x0004}---->{ n:0x0005}---->{n:{m:0x0006}}--->{n:{m:1}}

地址	变量	存储值
0x0001	a  (1)	n:0x0002  n:0x0004 ---(1)
0x0002	-n-  (1)	1
0x0003	b	0x0001
0x0004	n    (1)	0x0005
0x0005	a    (1)	m:0x0006    (2)
0x0006	m    (2)	1         (2)

var a = {n:1};
var b = a;
a={m:1};//a重新赋值,换了一个房间,b仍然指向原来的房间(地址)
a.n =a;
console.log(a);//{m:1,n:{m:1,n:{..}}}//一直循环
console.log(b);//{n:1}

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