In JavaScript, when an associative array a is assigned to b, and then the content of a is changed, why does b also change?

<code>JS:
var a = {"Client":"jQuery","Server":"PHP"};
var b = a;
a["New"] = "Element";
console.log(b);
// 输出 Object { Client="jQuery",  Server="PHP",  New="Element"}

PHP例程1:
$a = array('Client'=>'jQuery','Server'=>'PHP');
$b = $a;
$a['New'] = 'Element';
var_export($b);
//输出 array('Client'=>'jQuery','Server'=>'PHP')

PHP例程2:
$a = array('Client'=>'jQuery','Server'=>'PHP');
$b = &$a; //引用赋值
$a['New'] = 'Element';
var_export($b);
//输出 array('Client'=>'jQuery','Server'=>'PHP','New'=>'Element')</code>

In JavaScript, when an associative array a is assigned to b, and then the content of a is changed, why does b also change?

Reply content:

<code>JS:
var a = {"Client":"jQuery","Server":"PHP"};
var b = a;
a["New"] = "Element";
console.log(b);
// 输出 Object { Client="jQuery",  Server="PHP",  New="Element"}

PHP例程1:
$a = array('Client'=>'jQuery','Server'=>'PHP');
$b = $a;
$a['New'] = 'Element';
var_export($b);
//输出 array('Client'=>'jQuery','Server'=>'PHP')

PHP例程2:
$a = array('Client'=>'jQuery','Server'=>'PHP');
$b = &$a; //引用赋值
$a['New'] = 'Element';
var_export($b);
//输出 array('Client'=>'jQuery','Server'=>'PHP','New'=>'Element')</code>

In JavaScript, when an associative array a is assigned to b, and then the content of a is changed, why does b also change?

For non-ordinary types such as arrays (strings, integers, Boolean), your assignment is equivalent to address copying, that is, a and b occupy the same address. So if b is changed, a will also change. Essentially, a and b are the same thing.

This answer here is more detailed. Arrays are reference type values ​​and are stored in the heap. https://www.zhihu.com/questio...

I saw someone on the Internet saying that when assigning values ​​in JS, original types (such as strings) are copied values, and reference types (such as associative arrays) are copied references.

<code>var a = {"Client":"jQuery","Server":"PHP"};
var b = JSON.stringify(a); //转成字符串后赋值
a["New"] = "Element";
console.log(JSON.parse(b)); //使用时转回关联数组(对象)
//输出 Object { Client="jQuery",  Server="PHP"}

IE8不支持JSON.parse和JSON.stringify,需要引入json2.js:
http://www.json.org/js.html
https://github.com/douglascrockford/JSON-js/blob/master/json2.js
IE9以下版本:
<!--[if lt IE 9]><script src="json2.js"></script><![endif]--></code>

I feel that JS arrays are not as flexible as PHP. PHP supports reference assignment using & declarations. PHP arrays are "copy-on-write":

<code>echo round(memory_get_usage()/(1024*1024))."MB\n"; //0MB
$a = file('/home/eechen/note.txt', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
echo round(memory_get_usage()/(1024*1024))."MB\n"; //9MB
$b = $a;
echo round(memory_get_usage()/(1024*1024))."MB\n"; //9MB(赋值后内存没有变化)
$b['new'] = 'element';
echo round(memory_get_usage()/(1024*1024))."MB\n"; //14MB(修改后内存发生变化,即写时复制)</code>

Because a and b both point to the same array.

If you want to keep it unchanged, first convert the object into a string, and then convert it back into an object. They will be two different objects. If used directly, it is actually one. The correct answer has been given above.