JS-Context

inspired by go's context package.

⤵️ Pass scoped data and functionality down your stack
🔐 Immutability guarantee
💪 Typescript support
📄 Zero dependencies
☑️ Unit Tested

Installation

$ npm install js-context

import {Context} from "js-context"

Context Creation

A Context, at its bare minimum, is just an immutable (frozen) object that you can pass around your functions.

//context creation
let ctx = new Context()
doSomething(ctx, ...otherParams)

Setting Values

You can use the .with() method on a context to set values. The .with function always returns a new context that inherits all properties from its parent. This allows for safety when passing the context deep into nested function calls, as there is no way to modify the context out of scope. This is the only way to set values on a context.

//setting values on a context
let ctx = new Context()
ctx = ctx.with("foo", "bar") //set a single value (key, value)
ctx = ctx.with({foo: "bar"}) //set multiple values (values)
console.log(ctx.foo) // => bar

If you set a value on a context that is already set, the value will be shadowed so that the new context cannot see the previous value, but the original context value will be unmodified.

const rootCtx = new Context()
const ctx1 = rootCtx.with("foo", 0)
const ctx2 = ctx1.with("foo", 1)
console.log(ctx1.foo) // => 0
console.log(ctx2.foo) // => 1

Merging Deep Values

When setting values that are objects, values will be deeply merged instead of being replaced. All plain objects have the same shadowing properties as described in the previous section.

const rootCtx = new Context()
const ctx1 = rootCtx.with({
    obj: { a: "stays", b: 0 }
})
const ctx2 = ctx1.with({
    obj: { b: 1 }
})
console.log(ctx1.obj.b) // => 0
console.log(ctx2.obj.b) // => 1
console.log(ctx2.obj.a) // => stays

//Object values actually get turned into context objects internally:
console.log(ctx1.obj instanceof Context) // => true
console.log(ctx2.obj instanceof Context) // => true

Symbol keys

If you need an object on the context that maintains reference equality, use a symbol for the key. JS-Context will not do any deep merging with these values. This is useful for modules who need to maintain complex state, or values that should not be directly modifiable by the context user.

const sym = Symbol("example symbol")
const init = {state: 0}
let ctx = new Context().with(sym, init)

console.log(ctx[sym].state) // => 0
console.log(ctx[sym].state === init) // => true

ctx[sym].state = 1 //valid because key is a symbol, therefore state is the original unmodified object
console.log(ctx[sym].state) // => 1
console.log(init.state) // => 1

ctx = ctx.with(sym, {other: 2}) //shadows the entire value (no deep nesting)
console.log(ctx[sym].other) // => 2
console.log(ctx[sym].state) // => undefined

Functions On Context

You can attach functions to a context using withCtxFunction. Use this to add utility functions (such as logging) to the context. The first argument to the function will always be the current context. Alternatively you can use with(key, function(){}) and the current context will be this.

let ctx = new Context()
ctx = ctx.with("logLevel", 4)
ctx = ctx.withCtxFunction("log", (ctx, ...args) => {
    if(ctx.logLevel > 3) {
        console.log(...args)
    }
})
ctx.log("Hello World") // => Hello World

Example using this

//accessing ctx via this
ctx.with("log", function(...args) {
    if(this.logLevel > 3) {
        console.log(...args)
    }
})
ctx.log("Hello World") // => Hello World

Performance

Context Deep Freezing

To guarantee immutability, JS-Context freezes all objects on the context by default.
It is recommended to disable this feature when in production by using setShouldFreeze(false). Freezing has been known to cause performance issues in certain javascript engines.

let ctx = new Context()
ctx.foo = "bar" // => Error: ctx is frozen

//objects get deeply frozen
ctx = ctx.with({obj: {a: 0}})
ctx.obj.a = 1 // => Error: obj is frozen

Prototype chain

⚠️ Details for nerds ⚠️
JS-Context uses prototype inheritance to allow for the "shadowing" behavior without making copies of the data every time you use with. However, this comes at the cost of runtime lookup performance. When you access a property, the javascript engine has to walk all the way up the prototype chain until it finds the property its looking for. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain for more information on prototype chains.
If you have a lot of properties to add, and you want to avoid this performance cost, use the ContextBuilder. It avoids the prototype inheritance by putting all the properties on the same object.

Modules

Modules are libraries that expose functionality on a context via the .with() methods. Some of these expose functions as properties on the context, other functions must be called, passing a context.

Cancel

The cancel module allows you to add cancellation to a context. This allows you to conditionally exit early lower in your stack.

import {Context} from "js-context"
import {withCancel, cancelContext} from "js-context/cancel"

let ctx = withCancel(new Context())

(async () => {
    try {
        const taskResult = Promise.race([
            someLongRunningTask(ctx, ...otherParams), 
            ctx.whenCancelled() //rejects an Error with message specified from cancelContext
        ])
    } catch (err) {
        console.error(err)
    }
})

//...somewhere later down the line
cancelContext(ctx, "some detailied description about why it was cancelled")

If you just need a simple timeout cancellation, you can do so via withCancelTimeout. It behaves the exact same way as a cancellation context, except it has a setTimeout for the cancelContext call. If you ever need to cancel this timeout for whatever reason (maybe trying to empty the event loop), you can call cancelTimeout.

import {Context} from "js-context"
import {withCancelTimeout, cancelContext} from "js-context/cancel"

let ctx = withCancelTimeout(new Context(), 10_000) //10 seconds

(async () => {
    try {
        await ctx.whenCancelled() //rejects an Error with a timeout message after 10 seconds
    } catch (err) {
        console.error(err)
    }
})

Store

Store a Map on the context that will is shared between all child contexts. You can use this as a cache for data between different parts of your program.

import {Context} from "js-context"
import {withStore} from "js-context/store"

const ctx = withStore(new Context())
const store = ctx.getStore() //this is a Map that is shared between all child contexts.

Other Modules:

name	description
winston	Contextual winston logging
aws-lambda	AWS lambda timeout and AWS context information
knex	Adds knex to context
mssql	Adds mssql to context

Submit a pull request to have your module listed here! 😄

Module Requirements:

• Must have tests
• Must have typescript types
• Methods that add to context follow convention withXYZ(ctx)