a tiny userland registry, using a "parse" and "link" primitive

a.js

import {b} from "./b.js";
import $ from "jquery";

export function a() {
  // ...
}

b.js

import {a} from "./a.js";
import $ from "jquery";

export function b() {
  // ...
}

tiny-example.js

class TinyLittleRegistry {
  constructor(realm, jquery) {
    this.#realm = realm;
    this.#jquery = jquery;
    this.links = {
      "a": {
        "./b.js": "b",
      },
      "b": {
        "./a.js": a
      },
    };
  }

  @lazy records() {
    return Promise.all([fetch("a.js"), fetch("b.js")])
                  .then(sources => sources.map(s => this.#realm.parseModule(s)));
  }

  @lazy async graph() {
    const { a, b } = await this.records();
    const { jquery } = this; 
    return { a, b, jquery };
  }

  getDependencyOf(referrer, specifier) {
    const modules = await this.#registry.graph();
    const name = Object.keys(modules).findIndex(name => modules[name] === referrer);
    if (!name) {
      // must be a top level
      return module[specifier];
    }
    const depName = this.link[name][specifier];
    return depName ? modules[depName] : Promise.resolve(new ReferenceError(`unrecognized module: ${depName} from referrer ${name}`));
  }

  getTopLevelModule(name) {
    return this.#registry.graph().then(graph => graph[name]);
  }
}

import * as jquery from "jquery";

class TinyLittleRealm extends Realm {
    constructor() {
      super();
      this.#registry = new TinyLittleRegistry(this, jquery);
    }

    [Realm.import](name, referrer) {
      if (!referrer) {
        return this.#registry.getTopLevelModule(name);
      }
      return this.#registry.getDependencyOf(referrer, name)
    }
}

let realm = new TinyLittleRealm();

realm.eval("import('a')").then(m => m.a());