为什么“导出默认常量"无效?

问题描述

我认为以下内容很好:

const Tab = connect( mapState, mapDispatch )( Tabs );导出默认选项卡；

然而，这是不正确的:

export default const Tab = connect( mapState, mapDispatch )( Tabs );

不过这很好:

export default Tab = connect( mapState, mapDispatch )( Tabs );

能否解释一下为什么const 对export default 无效?它是一个不必要的添加 &任何声明为 export default 的东西都假定为 const 之类的?

解决方案

const 就像 let, 它是一个 LexicalDeclaration (VariableStatement, Declaration) 用于在块中定义标识符.

您试图将其与 default 关键字混合使用，它期望 HoistableDeclaration、ClassDeclaration 或 AssignmentExpression 跟随它.>

因此它是一个SyntaxError.

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如果你想const，你需要提供标识符而不是使用default.

export 本身接受一个 VariableStatement 或 Declaration 在它的右边.

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以下就可以了export default Tab;

Tab 变成了 AssignmentExpression，因为它的名字是 default

export default Tab = connect( mapState, mapDispatch )( Tabs ); 没问题

这里Tab = connect( mapState, mapDispatch )( Tabs ); 是一个AssignmentExpression.

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更新:想象问题的不同方式

如果您试图从概念上理解这一点并且上面的规范推理没有帮助，请将其视为如果 default 是合法标识符而不是保留标记，编写 export default Foo; 和 export default const Foo = 1; 的不同方式是什么?"

在这种情况下，扩展的编写方式是

//伪代码，这个思想实验是无效的JS导出默认 Foo；//会像导出常量默认值 = Foo;导出默认 const Foo = 1;//会像导出 const 默认 const Foo = 1;//那么下面这行有意义吗?const bar const Foo = 1;

有一个有效的论证，扩展应该类似于

//伪代码，这个思想实验是无效的JS导出默认 const Foo = 1;//会像常量 Foo = 1;导出常量默认值 = Foo;

但是，根据 Sergey 的说法，这会变得模棱两可注释，所以明确地写这个模式更有意义.

I see that the following is fine:

const Tab = connect( mapState, mapDispatch )( Tabs );
export default Tab;

However, this is incorrect:

export default const Tab = connect( mapState, mapDispatch )( Tabs );

Yet this is fine:

export default Tab = connect( mapState, mapDispatch )( Tabs );

Can this be explained please why const is invalid with export default? Is it an unnecessary addition & anything declared as export default is presumed a const or such?

解决方案

const is like let, it is a LexicalDeclaration (VariableStatement, Declaration) used to define an identifier in your block.

You are trying to mix this with the default keyword, which expects a HoistableDeclaration, ClassDeclaration or AssignmentExpression to follow it.

Therefore it is a SyntaxError.

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If you want to const something you need to provide the identifier and not use default.

export by itself accepts a VariableStatement or Declaration to its right.

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Tab becomes an AssignmentExpression as it's given the name default

Here Tab = connect( mapState, mapDispatch )( Tabs ); is an AssignmentExpression.

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Update: A different way to imagine the problem

If you're trying to conceptually understand this and the spec-reasoning above is not helping, think of it as "if default was a legal identifier and not a reserved token, what would be a different way to write export default Foo; and export default const Foo = 1; ?"

In this situation, the expanded way to write it would be

// pseudocode, this thought experiment is not valid JS

export default Foo;
// would be like
export const default = Foo;

export default const Foo = 1;
// would be like
export const default const Foo = 1;
// so would the following line make sense?
const bar const Foo = 1;

There is a valid argument the expansion should be something like

// pseudocode, this thought experiment is not valid JS

export default const Foo = 1;
// would be like
const Foo = 1;
export const default = Foo;

However, this then would become ambiguous per Sergey's comment, so it makes more sense to write this pattern explicitly instead.

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