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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
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。

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
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?

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