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

var arr = [1,2];
var arr1 = arr;
arr = [1,3];
console.log(arr1);//[1,2];    
//arr = [1,3]实际上是在堆内另建了一个地址,arr指向了新的地址,而arr1还是指向原来的地址,故arr1值不变

要避免使用连续赋值符号:下面举两个例子

 a = {n:1 b == a={m:1console.log(b);

1.var a = {n:1},在内存中申请两个地址,分别存放a和n;

2.var b = a;申请一个地址存放b,值指向a的地址;

3.a.n= a={m:1};

(1)(这里对变量进行了赋值,因此要重新申请内存)先申请了内存地址n(0x0004)和a(0x0005);由于n值占用了新的内存,故0x0001处 存储值由n:0x0002变为n:0x0004;

(2)赋值;a={m:1},申请m地址(0x0006),

因此:a : {m:0x0006}--->{m:1}

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

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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