Added type-level mapping between aliases and nodes that have that alias.

[cameron-martin]

Q	A
Fixed Issues?	Fixes #9092
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	👍
Tests Added + Pass?	No
Documentation PR Link
Any Dependency Changes?
License	MIT

Problem

The TypeScript types for Visitor in @babel/traverse do not allow node aliases as properties. These cannot be added with the current contents of the auto-generated TypeScript definitions for @babel/types.

Solution

If @babel/types had a type-level mapping between aliases and the node types that have those aliases, e.g:

export interface Aliases {
  Function: Function;
  Expression: Expression;
  LVal: LVal;
  // ...etc
}

Then the Visitor type in the TypeScript definitions can be intersected with the following type:

{ [K in keyof Aliases]?: VisitNode<S, Aliases[K]> }

This would then allow using aliases as Visitor properties when using TypeScript.

This PR adds such an Aliases type.

Concerns

• There are no tests for this, because it seems like the infrastructure for testing d.ts files is not set up. If tests are required then I can set this infrastructure up.
• This does not add equivalent functionality to the flow types. I believe that if the same thing was added, then it could be used in the same way as the TypeScript types by using $ObjMap. Can somebody confirm what the situation with flow definitions for @babel/traverse are and whether this would be a useful addition to the flow types?

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/9504/

[danez]

Fine with me

[cameron-martin]

What's the process for merging PRs? Specifically, how come this has been approved but not yet merged?