async-context-tc39

TC39 proposal implementation for AsyncContext.

Summary

This is an implementation of the proposal https://github.com/tc39/proposal-async-context that works with JavaScript native awaits in browser. All the awaits on the path between where the context value is set and where it is used should be wrapped into Ѧ-functions to capture and restore the current context (that is required because the awaits use a native mechanism that is not interceptable/hookable at all). Use eslint-plugin-async-context-tc39 that will search for all the await expressions not wrapped into the Ѧ-functions in your project and fix them.

Most of the tests were copied from the project https://github.com/iliasbhal/simple-async-context (the TC39 implementation that requires the code to be built to ES6).

Install

npm install async-context-tc39

Usage:

import { AsyncContext } from 'async-context-tc39';

const asyncContext = new AsyncContext.Variable<string>();

//...
  it("test", async () => {
    const deepCallback = async () => {
      expect(asyncContext.get()).toBe("Inner");
      Ѧ(await wait(30).Ѧ);
      expect(asyncContext.get()).toBe("Inner");
    };

    const innerCallback = () => asyncContext.run("Inner", async () => {
      expect(asyncContext.get()).toBe("Inner");
      Ѧ(await wait(30).Ѧ);
      expect(asyncContext.get()).toBe("Inner");
      Ѧ(await deepCallback().Ѧ);
      expect(asyncContext.get()).toBe("Inner");
      Ѧ(await wait(30).Ѧ);
      expect(asyncContext.get()).toBe("Inner");
    });

    const total = () => asyncContext.run("Outer", async () => {
      expect(asyncContext.get()).toBe("Outer");
      Ѧ(await innerCallback().Ѧ);
      expect(asyncContext.get()).toBe("Outer");
      Ѧ(await innerCallback().Ѧ);
      expect(asyncContext.get()).toBe("Outer");
      Ѧ(await innerCallback().Ѧ);
      expect(asyncContext.get()).toBe("Outer");
    });

    expect(asyncContext.get()).toBe(undefined);
    Ѧ(await total().Ѧ);
    expect(asyncContext.get()).toBe(undefined);
  });

Classes

namespace AsyncContext {
  class Variable<T> {
    constructor(options: AsyncVariableOptions<T>);
    get name(): string;
    get(): T | undefined;
    run<R>(value: T, fn: (...args: any[])=> R, ...args: any[]): R;
    wrap<Fn extends (...args: any) => any>(value: T, callback: Fn): Fn;
  }

  interface AsyncVariableOptions<T> {
    name?: string;
    defaultValue?: T;
  }

  class Snapshot {
    constructor();
    run<R>(fn: (...args: any[]) => R, ...args: any[]): R;
    static wrap<T, R>(fn: (this: T, ...args: any[]) => R): (this: T, ...args: any[]) => R;
  }
}

See README.md on https://github.com/tc39/proposal-async-context for datails