asimo - asynchronous dependency manager

Asimo is a micro libray that helps managing JS objects & modules dependencies (e.g. to share/retrieve objects or modules in a JS/TS code base). Asimo approach is different from traditional solutions like Dependency Injection (DI) libraries as its principle is to retrieve dependencies on-demand and asynchronously (which also allows to load the dependency module on-demand, thus the name ASYnchronous MOdule loader).

Asimo was built from the following realizations:

• Advanced testing requires a generic mechanism to replace code dependencies in test environments (e.g. to use a fake localStorage service to simulate different start state in an application).
• Dependency Injection is not well suited to progressive code load, such as in client applications where large code bases cannot be loaded on application startup. With Dependency injections, all dependencies are loaded when a given entity is created which eventually results in large code packages (unless significant effort is made to prevent that issue). On the contrary, asimo approach is to load dependencies on-demand, which allows to reference rarely used functionalities anywhere in the application (with their type) without impacting the initial download, as the actual implementation will be loaded on-demand. Besides asimo allows to group JS modules into bundles independently from the code structure (in other words the code doesn't need special attention to support efficient progressive load)

Why should you use asimo?

Asimo helps solving 4 types of problems:

• unit testing: asimo can replace dependencies with mock dependencies (e.g. in the test enviroment, unit tests can call a fake fetch() that will return stable data).
• development: asimo simplifies the development of mock environment solutions that simulate and synchronize multiple external dependencies (e.g. fetch(), localStorage, SSE events, etc.). The mock environment can define multiple profiles that will correspond to different datasets / behaviours. On top of that, the mock environment code can be loaded dynamically as an asimo bundle (i.e. through a dynamic import) and won't be packaged with the rest of the application. Last but not least this mock environment (mockenv) can be used for unit tests, development, demos and also for full client integration tests with tools like Playwright - more in the asidemo example
• application code splitting: asimo allows to package the application into several bundles that will be downloaded on-demand, when one of its modules is required. This allows to improve the application startup time by optimizing the initial application load. Besides the application code is not aware of the bundle configuration that can be changed without any code refactoring.
• dependency sharing: asimo can be used to implement a React-like context outside a JSX environment (as React context can only exist in a DOM-related environment). This allows to provide losely-coupled (but still typed) dependencies through a unique context object and comes in handy to reduce the number of variables to pass to the different parts of the code base.

Live Demos: 🚀 sample app architecture (asidemo) or google search results

Presentation slides

Other key features:

• design by interface: depend on abstraction, not implementation - cf. SOLID design principles
• full and transparent typescript support
• small library: less than 200 lines of code.

How does it work?

Like most dependency management systems, Asimo base principle is to create dependency context objects that will be called by the application to retrieve dependencies. A typical dependency retrieval will look like this:

// retrieve the calculator service (asynchronous)
const calc = await asm.get(CalculatorIID);
//     CalculatorIID is the Calculator Interface Identifier
//     asm is an Asimo dependency context
//     calc is a Calculator service instance

// use it!
const result = calc.add(1, 2);
//     -> the service doesn't need to be async

The Dependency contexts are objects on which dependency factories can be registered - e.g.

// Service registration
asm.registerService(CalculatorIID, () => new CalculatorService());
//     of course CalculatorService must implement the interface associated
//     to CalculatorIID --> typescript will help you there in case of error

Asimo supports 3 kinds of factories

• service factories (cf. registerService()) that produce unique objects that will be shared by application modules - i.e. the service instance will be created on the first get() call, and then asimo will always return this instance instead of creating a new one.
• object factories (cf. registerFactory()) that produce new object instances everytime they are called.
• group factories (cf. registerGroup()) that are used to declare bundles (i.e. multiple JS modules that will be packaged together).

On top of that, asimo also supports registering any kind of objects. Note: in this case the retrieval methods are synchronous (cf. registerObject(), getObject() and fetchObject())

// Object registration
const calc = new CalculatorService();
asm.registerService(CalculatorIID, calc);

const calc2 = asm.getObject(CalculatorIID);
// calc2 is of type  Calculator (cf. below) and is equal to calc:
expect(calc2).toBe(true);

Last but not least, and like with any dependency management systems, asimo allows to chain dependency contexts by creating sub-contexts - e.g.

const context2 = asm.createChildContext("context2");
// context2 is a name that will help differentiate context2 from asm in error messages

// another method - equivalent to context2.createChildContext("context3");
const context3 = createContext({ name: "context3", parent: context2 });

Note: by default asimo creates a root context that is exposed as asm by the asimo module (i.e. @asimojs/asimo)

In this example, if an application module tries to retrieve a dependency on context3 asimo will first look in context3, then in context2 and then in asm if it hasn't found it before. Asimo offers 2 methods to manage cases where dependency factories are not found (i.e. not registered):

• either it throws an exception as the error results from a configuration issue that can only occurs at development time because the factory registration is missing. The benefit is that the dependency user doesn't need to check the object returned by asimo. This behavior is implemented by the get() method.
• or it returns null and the dependency user will have to manage the case where the returned object is not available (typescript will show that the type can be null). This behavior is implemented by the retrieve() method.

The last special part that needs to be mentioned concerns the interface definitions. As typescript interfaces cannot be read at runtime, asimo exposes a special interfaceId() method that allows to bind a typescript interface with a string (that should be unique - this is why we suggest to use namespaces):

// typescript interface
export interface Calculator {
    numberOfCalls: number;
    add(a: number, b: number): number;
}

// interface id token associated to the typescript interface
export const CalculatorIID = interfaceId<Calculator>("asimo.src.tests.Calculator");
// asimo.src.tests.Calculator -> unique namespace (by construction)

Examples

In all use cases asimo involves at least 2 modules:

• a consumer module that retrieves the producers dynamically, on-demand (the consumer can then keep a reference to its dependencies to avoid further async calls if need be):

// Consumer
import { asm } from "@asimojs/asimo";
import { CalculatorIID } from "./types";

async function doSomething() {
    const calc = await asm.get(CalculatorIID);
    // Typescript sees that calc is of type Calculator
    const result = calc.add(1, 2); // 3
}

• a producer module that contains services or factories that will be exposed to consumers. Producers are usually split in 2 files:

-- a type file:

import { interfaceId } from "@asimojs/asimo";

// typescript interface
export interface Calculator {
    numberOfCalls: number;
    add(a: number, b: number): number;
}

// interface id token that associates a string namespace
// (the runtime token) to the typescript interface
export const CalculatorIID = interfaceId<Calculator>("asimo.src.tests.Calculator");

-- and an implementation file:

// Producer (implementation file)
import { asm } from "@asimojs/asimo";
import { Calculator, CalculatorIID } from "./types";

// Calculator implementation
export class CalculatorService implements Calculator {
    numberOfCalls = 0;

    add(a: number, b: number) {
        this.numberOfCalls++;
        return a + b;
    }
}

// Service registration
asm.registerService(CalculatorIID, () => new CalculatorService());

Install

npm i @asimojs/asimo
# or
yarn add @asimojs/asimo

APIs

Asimo core module exports 3 entities:

• asm - the root AsmContext that exposes most asimo APIs
• createContext() - that allows to create asimo contexts
• interfaceId() - that allows to create interface id objects that associate interface namepsaces with typescript types:

function interfaceId<T>(ns: InterfaceNamespace): InterfaceId<T> {...}

/**
 * String representing an interface namespace - e.g. "myapplication.services.Settings"
 */
export type InterfaceNamespace = string;

/**
 * Object biding an interface type T with an interface namespace (allows to get typescript validation)
 */
export interface InterfaceId<T> {
    ns: InterfaceNamespace;
}

Examples:

export interface Multiplier {
    multiply(a: number, b?: number): number;
}
export const MultiplierIID = interfaceId<Multiplier>("asimo.src.tests.Multiplier");

interface Adder {
    (a: number, b: number): number;
}
export const AdderIID = interfaceId<Adder>("asimo.src.tests.Adder");

AsmContext

The AsmContext is the object that contains the registered factories and that exposes all asimo APIs (but interfaceId()):

get()

Retrieve a service or an object instance. For each interface id, asimo will first look in the current context for services or object factories or groups (in this order) - if not found it will then perform the same lookup in its parent context, recursively up to the root context). This method allows to retrieve up to 5 dependencies in one call with type support (more can be retrieved without type inference - cf. call signature).

Note: get() will throw exceptions if the factory is not defined for the expected interface. This is usually the preferred behavior as a missing factory generally results from an application configuration issue that is fixed at development time. Thanks to this behavior the dependency user doesn't need to check the type returned by asimo.

Note 2: the parameters can be either InterfaceId objects or interface namespaces (strings). When using InterfaceId typescript will automatically infer the right type - otherwise an explicit type cast will be necessary, as shown below

Examples:

// Get one dependency
const calc = await asm.get(CalculatorIID);
calc.add(1, 2); // calc2 type is Calculator

// Get one dependency by namesapce
const calc2 = await asm.get("asimo.src.tests.Calculator");
(calc2 as Calculator).add(1, 2); // calc2 type is unknown

// Retrieve multiple dependencies - also works with namespace strings:
const [m, c, a] = await asm.get(MultiplierIID, CalculatorIID, AdderIID);

fetch()

This method is similar to get() with the difference that it will return null if the dependency is not found (instead of throwing an exception).

// Get one dependency
const calc = await asm.fetch(CalculatorIID);
calc?.add(1, 2); // calc2 type is Calculator | null

// Retrieve one dependency by namesapce
const calc2 = await asm.fetch("asimo.src.tests.Calculator");
(calc2 as Calculator)!.add(1, 2); // calc2 type is unknown

// Retrieve multiple dependencies - also works with namespace strings:
const [m, c, a] = await asm.fetch(MultiplierIID, CalculatorIID, AdderIID);
//     m is of type Multiplier
//     c is of type Calculator
//     a is of type Adder

registerService()

Register a Service (i.e. a singleton object or function). The service doesn't need to be instantiated at registration time as we need to provide a factory function (synchronous or asynchronous). Note: service factories are only called once whereas object factories are called at each get() (or fetch()) in order to generate new object instances.

registerService<T>(iid: InterfaceId<T>, factory: () => T | Promise<T>): void;

Examples:

// Register an object - factory will only be called once
asm.registerService(CalculatorIID, () => new CalculatorService());
// Registration with async factory
asm.registerService(MultiplierIID, async () => new MultiplierService());
// Register a function - cf. above
asm.registerService(AdderIID, () => add);
//     m is of type Multiplier | null
//     c is of type Calculator | null
//     a is of type Adder | null

registerFactory()

Register an object factory. The factory will be called any time the get method is called for the object's interface id. On the contrary to services, there is no restriction on the number of objects that can be creatd. Besides asimo doesn't keep any reference to the object

registerFactory<T>(iid: InterfaceId<T>, factory: () => T | Promise<T>): void;

Example:

// Register an object factory
asm.registerFactory(MultiplierIID, () => new MultiplierImpl());

registerObject()

Register any kind of object in the given context.

interface SimpleObject {
    name: string;
    increment(value: number): number;
}
const SimpleObjectIID = interfaceId<SimpleObject>("asimo.test.objects.simple-object");

const o: SimpleObject = {
    name: "foo",
    increment(v) {
        return v + 1;
    },
};
context.registerObject(SimpleObjectIID, o);

// retrieval is synchronous for objects (no await)
const o2 = context.getObject(SimpleObjectIID); // o2 type is SimpleObject
expect(o2).toBe(o);
const o3 = context.fetchObject(SimpleObjectIID); // o3 type is SimpleObject | null
expect(o3).toBe(o);

getObject()

Get an object from the given context (or its parent). This function will throw an error if no object is defined (on the contrary to fetchObject()) - cf. previous example.

fetchObject()

Get an object from the given context (or its parent). This function will return null if no object is defined (on the contrary to getObject()) - cf. previous example.

registerGroup()

Register a group loader that will be used to asynchronously load multiple services and object factories gathered in a same deployment bundle. The bundle code will be loaded on-demand, when an explicit get()/fetch() is done on one of its service or object interfaces.

Parameters:

• iids: the list of interface ids that are packaged in this group
• loader: an async factory that should dynamically import() the required modules

registerGroup(iids:InterfaceId<any>[], loader: () => Promise<unknown>): void;

Example:

// Register several interface implementation that will be loaded on-demand through
// a dynamic module import (this assumes that the implementaion are defined in the module)
asm.registerGroup([Service1IID, Object2IID], () => import("./groups/groupA"));

createChildContext()

Create a child context that can override some of the dependencies defined in its parent (cf. get behaviour)

createChildContext(name?:string): AsmContext;

Example:

import { asm as rootAsm } from "@asimojs/asimo";

function createContext() {
    const c = rootAsm.createChildContext();
    // override calculator service
    c.registerService(CalculatorIID, () => new CalculatorService());
    c.registerService(SyncIncrementorIID, () => new SyncIncrementorService());
    c.registerService(AsyncIncrementorIID, () => new AsyncIncrementorService());
    return c;
}

parent

Return the parent context associated to the current context - or null if the context is a root context (no parents).

name

Return the context name - helps differentiating multiple contexts. A unique context name is created if none is profided when creating the context through createContext() or createChildContext(). Note: the default root contex name is asm.

path

Return a unique path identifier listing all context names from the top parent context - e.g. "/asm/subContext1/subContext2". Useful to debug cases involving multiple contexts.

definitions

Return the list of interfaces defined in a given context. Interfaces are described through AsmInterfaceDefinition objects:

export interface AsmInterfaceDefinition {
    /** The interface id */
    iid: string;
    /** The type of of object the interface is going to produce */
    type: "service" | "object" | "group";
    /** Tell if the service associated to the interface has been loaded (not used for object or groups) */
    loaded?: boolean;
}

Example:

const defs = asm.definitions;
// defs = [
//     { iid: "asimo.src.tests.groups.Service1", type: "service", loaded: true },
//     { iid: "asimo.src.tests.AsyncIncrementor", type: "service", loaded: false },
//     { iid: "asimo.src.tests.Multiplier", type: "object" },
//     { iid: "asimo.src.tests.groups.Service1", type: "group" },
// ]

consoleOutput

Tell asimo how console logs should be handled - 2 possible values: "" or "Errors"

• "": no logs
• "Errors": log errors (e.g. invalid arguments or invalid resolution) [default]

Note: this setting is global, even if set on a child context. Example:

asm.consoleOutput = ""; // deactivate error logs
// [...]
asm.consoleOutput = "Errors"; // reactivate error logs