變量、綁定與手柄

對象常量的屬性能夠改變的？

｀值常量看似是「值不能變」意思，事實符號綁定不變，由於變量只是個手柄（地址引用），非容器

｀對象常量，不是對象內容不變，是對象引用（手柄）不能變

Object Declarations with const A const declaration prevents modification of the binding, not of the value.

const聲明防止綁定的修改，而不是值的修改

That means const declarations for objects don’t prevent modification of those objects. For example:

const person = {
name: "Nicholas"
};

// works 改內容不出錯
person.name = "Greg";

// throws an error 改綁定出錯
person = {
name: "Greg"
};
複製代碼

變量與符號綁定技術

1 符號綁定一個計算結果

To catch and hold values, JavaScript provides a thing called a binding, or variable:

let caught = 5 * 5;複製代碼

The previous statement creates a binding called caught and uses it to grab hold of the number that is produced by multiplying 5 by 5.

2 符號能夠解綁定

When a binding points at a value, that does not mean it is tied to that value forever. The = operator can be used at any time on existing bindings to disconnect them from their current value and have them point to a new one.

let mood = "light";
console.log(mood);
// → light
mood = "dark";
console.log(mood);
// → dark複製代碼

3 可見符號變量只是一個引用／手柄，而非容器

You should imagine bindings as tentacles, rather than boxes. They do not contain values; they grasp them—two bindings can refer to the same value. A program can access only the values that it still has a reference to. When you need to remember something, you grow a tentacle to hold on to it or you reattach one of your existing tentacles to it.

符號綁定與變量概念的區別

一個技術實現的概念，例如編譯器，一個是編程的概念，是高級語言的應用概念

爲何須要符號綁定

• 中間計算
• 複雜的計算（計算集中，數據集中）都是多步的
• 符號能夠抽象，表達複雜多步計算，符號種類有變量，表達，函數和類對象等多種粒度