Javascript – Why Is `Export Default Const` invalid

constantsdefaultexportjavascriptscope

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?

Best Answer

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.

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.

The following is fineexport default Tab;

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

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

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

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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