VariableDeclaration.js

import traverse from "@babel/traverse";
import t from "@babel/types";

traverse(ast, {
    FunctionDeclaration: function (path) {
      // path.node(FunctionDeclaration)
      path.insertAfter(t.variableDeclaration("var", [
        t.variableDeclarator(
            t.identifier("__proto"),
            t.memberExpression(t.identifier("Item"), t.identifier("prototype"))
        )
      ]));
    },
});
/*
function Item() {}
var __proto = Item.prototype;
*/