A thread local approximation built on async hooks, written in TypeScript


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00May 25, 2018Mar 12, 2018Minified + gzip package size for @creditkarma/async-scope in KB


Async Scope

Async Scope is a library built on the relatively new Node feature Async Hooks. It provides a key-value store that follows simple scoping rules across async boundaries. What that means is if you have a function that spawns an async coputation that operates outside of the lexical scope of the invoking function you can share data between the two functions. The idea being that a child async context has access to all of the data defined in parent async contexts, but not to the data defined in child contexts.

Let's look at a convoluted example:

import { AsyncScope } from '@creditkarma/async-scope'

const asyncScope: AsyncScope = new AsyncScope()

setTimeout(() => {
    function childFunction() {
        asyncScope.get<number>('foo') // returns 6

    function parentFunction() {
        asyncScope.set('foo', 6)
        setTimeout(() => {
        }, 1000)

}, 500)

setTimeout(() => {
    asyncScope.get<number>('foo') // returns null
}, 3000)

Think about this in terms of building a service framework where you have a server receiving HTTP requests and clients sending out HTTP requests. There is data on incoming HTTP headers that needs to be propagated to all clients (tracing, authentication, other company/user specific data). A library could set the desired values to the Async Scope store and client libraries could read that data without the service/application developers having to take any responsibility for passing data from server to client.


$ npm install --save @creditkarma/async-scope

Constructing an Instance

When constructing a new instance there are three optional parameters nodeExpiration, purgeInterval and maxSize. The idea behind Async Scope is that data should be very short-lived. Depending on what you are storing memory foot print could be non-trivial if left running for a long period of time. These options configure expiration of data. nodeExpiration defines how long data in a particular context should be allowed to live, defaults to 5 seconds. The option purgeInterval is how often the store looks for and ejects expired data, defaults to 10 seconds. The final option maxSize configures how many objects to hold in the scope store, defaults to 10000 items. If the maxSize is reach older objects are ejected to make room for new ones.

import { AsyncScope } from '@creditkarma/async-scope'

const asyncScope: AsyncScope = new AsyncScope({
    nodeExpiration: 5000,
    purgeInterval: 10000,
    maxSize: 10000,

Basic KV Store API

The Async Scope store supports three operations: get, set and delete.


Sets a value to be read by the current scope or any child scope.

import { AsyncScope } from '@creditkarma/async-scope'

const asyncScope: AsyncScope = new AsyncScope()
asyncScope.set('foo', 5)


Read a value from the current scope or any parent scope. It works like the prototype chain. It will return the first value matching the given key it finds by searching the chain (closest parent with matching key).

import { AsyncScope } from '@creditkarma/async-scope'

const asyncScope: AsyncScope = new AsyncScope()


Remove the given key from the current scope. This is a recursive delete. If there is more than one matching key in the current scope chain then all will be deleted.

import { AsyncScope } from '@creditkarma/async-scope'

const asyncScope: AsyncScope = new AsyncScope()

Debugging API

These are methods for inspecting the async scope when things aren't behaving as expected.


Return an array representing the lineage of the current scope. Each scope has a unique id. This lists, in order, the parents of the current scope. The current scope is the first element in the array, the next is the immediate parent and so on.

import { AsyncScope } from '@creditkarma/async-scope'

const asyncScope: AsyncScope = new AsyncScope()


Returns an object describing the size of the current store.

interface ISizeProfile {
    size: number
    maxSize: number

The object contains two properties. First size is the current number of scopes being tracked by the store. Second maxSize is the maximum number the store will hold before it starts ejecting old scopes.

import { AsyncScope } from '@creditkarma/async-scope'

const asyncScope: AsyncScope = new AsyncScope()


For more information about contributing new features and bug fixes, see our Contribution Guidelines. External contributors must sign Contributor License Agreement (CLA)


This project is licensed under Apache License Version 2.0

If you find any bugs or have a feature request, please open an issue on github!

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