理解Javascript不可变变量 [英] Understanding Javascript immutable variable

问题描述

我试图理解Javascript不可变变量的含义。如果我能做到：

I am trying to understand what a Javascript immutable variable means. If I can do:

var x = "astring";
x = "str";
console.log(x); //logs str` , then why it is immutable?

我能想到的唯一答案（从CI的一点点知道）是var x是一个指向具有值astring的内存块的指针，在第二个语句之后，它指向另一个值为str的块。是这样吗？

The only answer I can think (from the little bit of C I know) is that var x is a pointer to a memory block with the value "astring", and after the 2nd statement it points to another block with the value "str". Is that the case?

还有一个奖励问题：我对Javascript的值类型感到困惑。引擎盖下的所有变量都是对象吗？偶数和字符串？

And a bonus question: I was confused by the value types of Javascript. Are all variables objects under the hood? Even number and strings?

推荐答案

值是不可变的; 变量不是;它们包含对它们（原始）值的引用。

Values are immutable; variables are not; they hold a reference to their (primitive) values.

三种基本类型 string，number和boolean 具有对应的类型，其实例是对象： String，Number，Boolean 。

它们有时被称为 包装类型 。

The three primitive types string, number and boolean have corresponding types whose instances are objects: String, Number, Boolean.
They are sometimes called wrapper types.

以下值为原始的：

• 字符串：你好


• 数字：6,3.14（ JavaScript中的所有数字都是浮点数）


• 布尔值：true，false


• null：通常明确分配


• undefined：通常是默认值（自动赋值）

• Strings: "hello"
• Numbers: 6, 3.14 (all numbers in JavaScript are floating point)
• Booleans: true, false
• null: usually explicitly assigned
• undefined: usually the default (automatically assigned) value

所有其他值都是对象，包括基元的包装器。

All other values are objects, including wrappers for primitives.

所以：

• 默认情况下对象是可变的


• 对象具有唯一标识并通过引用进行比较


• 变量保持对对象的引用


• 基元是不可变的


• 基元按价值进行比较，它们没有个人身份

• Objects are mutable by default
• Objects have unique identities and are compared by reference
• Variables hold references to objects
• Primitives are immutable
• Primitives are compared by value, they don’t have individual identities

你可能会发现 JavaScript Primitives的秘密生活一个很好的解释。

You might find The Secret Life of JavaScript Primitives a good explanation.

此外，在ES6中有一个新的 const 关键字，它创建一个只读的命名常量，它不能通过赋值来改变值，或者在脚本运行时重新声明。

Also, in ES6 there is a new const keyword, that creates a read-only named constant that cannot change value through assignment or be re-declared while the script is running.

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