Remove expression property from Function{Declaration,Expression}

[fisker]

expression property on FunctionDeclration,FunctionExpression, and ArrowFunctionExpreesion is deprecated in ESTree https://github.com/estree/estree/blob/master/deprecated.md#functions long time ago

[ota-meshi]

I'm sorry if I'm wrong, but I think the expression property is required for ArrowFunctionExpression.

https://github.com/estree/estree/blob/master/es2015.md#arrowfunctionexpression

[fisker]

Thanks! Realized that after I open this, so I put this into draft. And opened an issue about it in ESTree estree/estree#323

[fisker]

You are right ArrowFunctionExpression.expression is required.

[marijnh]

I feel it's likely that some users of the library are likely to be relying on this (either because their code is old, or because they found the property in the output and decided to use it). I know acorn-walk does, for one thing. As such, I do not think making this change in a non-major release would be a good idea.

[fisker]

To clarify, ArrowFunctionExpression.expression indicates the function body is an Expression which is not removed.

And Function{Declaration,Expression}.expression is always false.

I know acorn-walk does

Are you referring this line ?

acorn/acorn-walk/src/index.js

Line 266 in 82233bf

c(node.body, st, node.expression ? "Expression" : "Statement")

It should still work the same as before.

Anyway, I understand your concern, it's acceptable if you want to do this in a major release.