Before this PR I have to memoize the link from typing to implementation

path.isBindingIdentifier => virtualTypes["isBindingIdentifier"].checkType

Now they are placed together. However, the typings and implementations can not be merged because TS does not support this assertion in non-class functions, although they will be eventually injected to the NodePath prototype.

Can we simplify Wrapper to remove checkPath? (I'm not sure if this is possible, I try to understand the codes but they are a bit complicated)

Nope. The checkPath applies additional selection logic based on Wrapper#types.

Say if we have a ReferencedIdentifier visitor, Babel knows an Identifier could be a ReferencedIdentifier from Wrapper#types. Then it wraps the visitor so that the wrapped visitor will first run wrapper.checkPath, if the node is a ReferencedIdentifier, the wrapped visitor then applies the original visitor to the Identifier node, otherwise the visitor is inactivated.

Could we pass nodeType to

function wrapCheck(nodeType: String, fn: Function) {
  const newFn = function (this: unknown, path: NodePath) {
    if (path[`is${nodeType}`]()) {
      return fn.apply(this, arguments);
    }
  };
  newFn.toString = () => fn.toString();
  return newFn;
}

Or, we can make checkPath just the check method name (as a string), to avoid the unnecessary intermediate function:

import * as virtualValidators from "./virtual-types-validator.ts"

export const ForAwaitStatement: Wrapper = {
  types: ["ForOfStatement"],
  checkPath: virtualValidators.isForAwaitStatement,
};

function wrapCheck(wrapper: Wrapper, fn: Function) {
  const newFn = function (this: unknown, path: NodePath) {
    if (wrapper.checkPath.call(path)) {
      return fn.apply(this, arguments);
    }
  };
  newFn.toString = () => fn.toString();
  return newFn;
}