slow partial async context support in the browser

demo.js

//
// Infrastructure to emualate partially async context in browser
//

// Every context that can be used in parallel by id
const contexts = new Map();

// Getting the context
function getContext() {
  // We extract the context id of the context id from the stack trace.
  // 1. It's slow
  // 2. It's limited by `Error.stackTraceLimit`
  // 3. It's only supports what async stack trace support. So no trace inside
  //    `setTimout` callback for example
  // I use `password-*-password` named function to make it easy to trace
  const re = /password-(\d+)-password/;
  const { stack } = new Error();
  const contextId = stack.match(re)?.[1];

  return contexts.get(contextId);
}

// The function that allow run the action with context
async function run(action, { context }) {
  // We create an id for storing thw context
  const contextId = ((Math.random() * 10_000) | 0).toString(10);
  // And store it
  contexts.set(contextId, context);

  // We make sure it would be easy to extract the `contextId` from stack trace
  // by creating a named function as a wrapper
  const methodName = `password-${contextId}-password`;

  const runner = {
    async [methodName]() {
      return await action();
    },
  }[methodName];

  try {
    await runner();
  } finally {
    // When action is finished we delete the stored context regardless success
    // or failure
    contexts.delete(contextId);
  }
}

///
/// Actual usage
///

async function action(id) {
  console.log(id, getContext());
  await new Promise((resolve) => setTimeout(resolve, 100));
  // If it wouldn't `getContext` would return wrong result here for one fo the
  // calls
  console.log(id, getContext());
  await new Promise((resolve) => setTimeout(resolve, 100));
  console.log(id, getContext());
  await new Promise((resolve) => setTimeout(resolve, 100));
}

// We must run in parallel to show it works
await Promise.all([
  run(action.bind(undefined, 1), { context: "a" }),
  run(action.bind(undefined, 2), { context: "b" }),
]);

// console output:
// ===============
// 1 'a'
// 2 'b'
// 1 'a'
// 2 'b'
// 1 'a'
// 2 'b'