JS淬炼: Primitive vs. Object

Javascript有两种基本数据类型，Primitive和Object。Object是properties的聚合，其property可以是Object也可以是Primitive。Primitive只有value, 没有properties。Javascript有五种Primitive:

string
number
boolean
null
undefined

除了null和undefined，其余Primitive都有对应的Object封装，如Object String对应string。

Stack vs Heap

Javascript是一门动态语言，同一个变量在不同时候可能被赋予不同数据类型，比如前面是number而后面变成Object。所以Javascript变量值所占用的内存空间应该是可以动态变化的。

var a = 10;
a = {};

实际上，不管是Primitive还是Object，这些Javascript变量的值都存储在可以动态分配空间的heap上。而它在stack上保存的则是一个固定size的包含该值所在heap位置信息的引用（类似pointer）。JS engine通过对引用指向的修改来实现同一个变量指向不同的数据类型。

Javascript memory model

It's wrong to state that primitives are allocated from the stack and only objects are allocated from the heap. This is the biggest difference between C and JavaScript.

最新的JS engine做了很多优化，使得内存分配更为高效但也更为复杂，具体可参考该StackOverflow回答。

值传递 vs 引用传递

Primitive是值传递（passed by value），Object是引用传递（passed by reference）。

上面说到，Javascript变量在stack保存的是地址引用，为什么又说Primitive是值传递呢？Javascript中所有Primitive都是immutable的。在传递Primitive时，JS engine在heap上复制一个值相同的Primitive，然后把新变量的引用传递出去，这就是所谓JS中的值传递。对于Object，JS engine并不会复制一个新的Object，而是直接传递它的地址引用（与C++/Java中的引用传递类似）。

该引用能识别其指向的变量是Primitive还是Object，所以它不是一个简单的pointer，而是包含所指向变量类型信息的“智能引用”。

对Prototype影响

在Javascript中，子类修改父类的property会影响到所有继承该父类的子类。但这影响会根据父类property是属于Primitive还是Object而有微妙差别。我们设想有一个父类father，和两个继承了他的子类son1和son2。

var father = { 
    primitive: 1,
    object: {
        fruit: "APPLE" 
    }
};
var son1 = Object.create(father);
var son2 = Object.create(father);

Primitive Property

如果父类的property是Primitive，该property通过passed by value传递到子类。这时，子类修改该property不会影响到父类本身，更不会传递到其他子类上。

console.log("father's primitive is " + father.primitive);
console.log("son1's primitive is " + son1.primitive);
console.log("son2's primitive is " + son2.primitive);
// father's primitive is 1
// son1's primitive is 1
// son2's primitive is 1

son1.primitive = 2;

console.log("father's primitive is " + father.primitive);
console.log("son1's primitive is " + son1.primitive);
console.log("son2's primitive is " + son2.primitive);
// father's primitive is 1
// son1's primitive is 2
// son2's primitive is 1

Object Property

如果父类的property是Object，该property通过passed by reference传递到子类上。子类修改该property会通过父类传递到所有继承它的子类上。

console.log("father's fruit is " + father.object.fruit);
console.log("son1's fruit is " + son1.object.fruit);
console.log("son2's fruit is " + son2.object.fruit);
// father's fruit is APPLE
// son1's fruit is APPLE
// son2's fruit is APPLE

son1.object.fruit = "GRAPE";

console.log("father's fruit is " + father.object.fruit);
console.log("son1's fruit is " + son1.object.fruit);
console.log("son2's fruit is " + son2.object.fruit);
// father's fruit is GRAPE
// son1's fruit is GRAPE
// son2's fruit is GRAPE

Reference

Primitive value vs Reference value
How variables are allocated memory in Javascript?