async-local

](https://codecov.io/gh/dimichgh/async-local)

async_hooks based local storage aka thread-local concept in java world.

The module provides a single layer of local storage for async flow, which is more than enough for a big platform.

Install

npm install async-local -S

Usage

Express middleware example

const AsyncLocal = require('async-local');
const express = require('express');
const app = express();
app.use((req, res, next) => {
    AsyncLocal.run(next);
});

Storing/reading data

const AsyncLocal = require('async-local');
console.log(AsyncLocal.get('foo')); // >>> undefined
AsyncLocal.set('foo', 'bar'); // >>> throw error if no context is setup

AsyncLocal.run(ctx => {
    AsyncLocal.set('foo', 'bar');
    // or
    ctx.set('foo', 'bar');

    const promise = Promise.resolve();;

    AsyncLocal.run(ctx => {
        console.log(ctx.get('foo')); // >>> bar
        console.log(AsyncLocal.get('foo')); // >>> bar
        AsyncLocal.set('foo', 'qaz');
        console.log(ctx.get('foo')); // >>> qaz
        console.log(AsyncLocal.get('foo')); // >>> qaz

        // promise preserves current context for the caller
        promise.then(() => {
            console.log(ctx.get('foo')); // >>> qaz
            console.log(AsyncLocal.get('foo')); // >>> qaz
        });
    });

    // promise preserves current context for the caller
    promise.then(() => {
        console.log(ctx.get('foo')); // >>> bar
        console.log(AsyncLocal.get('foo')); // >>> bar
    });
});

Binding context

There are some edge cases when context is not preserved in async flow. For such cases, it makes sense to bind context to the function or emitter explicitly.

const EventEmitter = require('events');
const emitter = new EventEmitter();
AsyncLocal.bindEmitter(emitter);

const origFn = () => {
    console.log(AsyncLocal.get('foo')); // >>> bar
};

AsyncLocal.run(() => {
    AsyncLocal.set('foo', 'bar');
    const fn = AsyncLocal.bind(origFn);

    fn();
})