为什么隐式符号到字符串转换会导致 JavaScript 中的 TypeError？

Question

在ES6中Symbol上有一个.toString()，其中returns是Symbol的字符串表示，但想知道为什么'' + Symbol()不起作用（ 运行 这个表达式会抛出 TypeError ，我没想到）？后者只是在新的 Symbol 上调用 .toString() 并将其附加 (+) 到空字符串吗？

Answer 1

来自 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/toString.

The Symbol object overrides the toString method of the Object object; it does not inherit Object.prototype.toString(). For Symbol objects, the toString method returns a string representation of the object.

Answer 2

Is the latter just calling .toString() on a new Symbol and append (+) it to empty string?

实际上，符号不能隐式转换为字符串或数字，尽管有趣的是您可以将它们隐式转换为布尔值。

MDN actually has a section 关于其中一些陷阱：

Symbol type conversions

Some things to note when working with type conversion of symbols.

• When trying to convert a symbol to a number, a TypeError will be thrown (e.g. +sym or sym | 0).
• When using loose equality, Object(sym) == sym returns true.
• Symbol("foo") + "bar" throws a TypeError (can't convert symbol to string). This prevents you from silently creating a new string property name from a symbol, for example.
• The "safer" String(sym) conversion works like a call to Symbol.prototype.toString() with symbols, but note that new String(sym) will throw.

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此行为记录在 abstract ToString operation 下的规范中：

Argument Type: Symbol

Result: Throw a TypeError exception.

abstract ToNumber operation 也类似：

Argument Type: Symbol

Result: Throw a TypeError exception.

要将 Symbol 转换为不带 TypeError 的字符串，您必须使用 toString 方法或 String().