README.md

@alxcube/di-container

Simple but flexible type-safe dependency injection container for TypeScript applications.

Key Features

• Flexible Service Creation: Utilize factories to create services with injected dependencies and customizable configurations, enhancing flexibility in service instantiation.
• Interface-Implementation Binding: Seamlessly link interfaces with their respective implementations.
• Lifecycle Management: Control the lifecycle of services with support for transient, singleton, and request scopes.
• Circular Dependency Support: Handle circular dependencies gracefully.
• Contextual Resolution: Dynamically resolve dependencies based on the context, enhancing adaptability.
• Type Safety: Ensure type safety throughout the dependency injection process, enhancing code robustness and reliability.
• Error Handling: Detect and manage errors during service resolution with clear and informative error messages, facilitating debugging.
• Testing Support: Simplify unit testing by instantiating classes with injected dependencies using the container's features. Additionally, backup and restore container state for seamless testing.

Installation

npm i @alxcube/di-container

Glossary

This section explains the terms used in this documentation.

Service Container

Implementation of Dependency Injection Container pattern.

Service Resolution Context

Service resolution context is a special object available in service factories. This object provides methods for retrieving services that are dependencies of the service being created by the factory. It exists within the scope of a single root service request. Additionally, it provides methods to determine whether the current service is being resolved as a dependency for another service, as well as the current stack of service resolution.

See ServiceResolutionContext interface.

Service

Service - an object or value obtained from the container. In general, services are implementations of your application's interfaces, but the container allows storing values of any type.

Service Factory

Service factory is a callback function that will be invoked when a service is requested from the container. The sole argument of the function is the Service Resolution Context object, which allows obtaining dependencies of the constructed service. This function should return a value of the corresponding type.

Service Map

Service map is an auxiliary interface that represents the mapping between service keys and service types. It enables TypeScript to leverage type inference, making calls to container methods type-safe, and assists you in working with code hints from your IDE.

While any strings can be used as property names in this interface, it is recommended to name them according to the names of your application's interfaces.

Index signature ([key: string]: any;) should not be used, since it breaks type inference.

interface ServicesMap {}

Service Key

Service key is either a key of the service map interface or a class constructor. Services are registered and retrieved from the container using this key.

type ServiceKey<TServicesMap extends ServicesMap> =
  | keyof TServicesMap
  | Constructor<object>

Named Service Key

Object with two properties: service - the service key, and name - the service name.

interface NamedServiceKey<TServicesMap extends ServicesMap> {
  service: ServiceKey<TServicesMap>;
  name: string;
}

Service Token

Type alias for union of service key and named service key.

type ServiceToken<TServicesMap extends ServicesMap> =
  | ServiceKey<TServicesMap>
  | NamedServiceKey<TServicesMap>;

Constant Token

Constant token is an object with a single property 'constant', which can hold a value of any type. It is used to declare dependencies of a class that are not retrieved from the container and are passed directly to the class constructor in methods registerClassConfig(), implement() and instantiate().

Dependencies Tuple

Dependencies tuple is a special type of tuple whose members are service tokens, or constant tokens, from which values of the corresponding type are resolved. It is used for declaratively specifying the dependencies of a class constructor in the methods registerClassConfig(), implement(), and instantiate().

Usage

Create Service Map

First of all, you need to create a service map and specify in it the types of services that will be available in the container.

// TypesMap.ts
import type { ServicesMap } from "@alxcube/di-container";
import type { HttpClient } from "./HttpClient";
import type { BackendApiClient } from "./BackendApiClient";

// Extending of ServicesMap is not required, but recommended for clarity.
export interface TypesMap extends ServicesMap {
  HttpClient: HttpClient;
  "HttpClient[]": HttpClient[];
  BackendApiClient: BackendApiClient;
  applicationKey: string;
}

Creating Container

After declaring service map, you are now ready to create container instance:

// container.ts
import { Container } from "@alxcube/di-container";
import type { TypesMap } from "./TypesMap";

export const container = new Container<TypesMap>();

Resolving Services

There are several methods for resolving services from container.

Resolving Single Service

To retrieve a service from the container, you use the resolve() method. It takes the service key as an argument and returns a value of the corresponding type. If the key passed is key of service map, the method returns the corresponding type according to the map. If a constructor is passed as the key, an instance of the provided class will be returned. However, remember to register the appropriate service factory for the class; the container does not inherently know how to instantiate class instances.

The second argument of the method, name, allows you to retrieve a service with the corresponding name. If this parameter is omitted, the name "default" is implicitly used (the same applies to registration in the container).

If there is no service registered in the container with the given key or name, a RangeError will be thrown.

const httpClient: HttpClient = container.resolve("HttpClient");

// There is no need to declare types, since type inference works using types map
const paymentsApiHttpClient = container.resolve("HttpClient", "payment");

Resolving Array Of Services

The resolveAll() method takes a service key and returns an array of services of the corresponding type registered under different names. If there are no registrations for this service in the container, an empty array will be returned.

// The inferred type is HttpClient[]
const httpClients = container.resolveAll("HttpClient");

Resolving Tuple Of Services

To obtain a tuple of services, you use the resolveTuple() method. It takes a tuple as an argument, whose members are service tokens. The return value is a tuple of the corresponding services.

This method can be useful when you need to retrieve multiple services from the container within the same context, when the services have a 'request' lifecycle. Using this method for services with other lifecycles does not make sense. The method is also available on the service resolution context object (in service factories), but using it there does not make sense either, as calls to resolve() on the ServiceResolutionContext object will already return services within the context of the same request.

const [httpClient, backendApiClient] = container.resolveTuple([
  {
    service: "HttpClient",
    name: "backend"
  }, // Named services can be resolved, using NamedServiceKey interface
  "BackendApiClient"
] as const); // Don't forget "as const" to make type inference work

Retrieving Service Names

Using the getServiceNames() method, you can obtain an array of all names under which a service with the given service key has been registered. If the service was registered without explicitly specifying a name, this array will include the name "default". If the service was not registered, an empty array will be returned.

container.registerConstant("HttpClient", new ConcreteHttpClient());
container.registerConstant("HttpClient", new AnotherHttpClient(), { name: "another" });

console.log(container.getServiceNames("HttpClient")); // ["default", "another"]

Registering Services

There are several ways to register services.

Registering Constant

To register a constant value, you use the registerConstant() method. It takes a service key and the constant value as arguments. Values of any type are supported, except for undefined. For example, you can register a primitive value or a singleton object created outside the container using this method.

The third optional argument is an options object. The following options are available:

• name - Allows registering multiple services of the same type under the same service key. It serves to differentiate between services of the same type. If this option is not specified, the name "default" will be used implicitly.
• replace - When set to true, it replaces the service (taking into account the service name) that was previously registered. If the option is set to false or omitted, and a service with the given key (and name) is already registered, a TypeError will be thrown.

// Register string constant
container.registerConstant("applicationKey", "my_app_key");

// Register interface implementation as singleton
container.registerConstant("HttpClient", new ConcreteHttpClient(), { name: "payments" });

// Replace registered services
container.registerConstant("applicationKey", "other_app_key", { replace: true });
container.registerConstant("HttpClient", new ConcreteHttpClient(), { name: "payments", replace: true });

Registering Service Factory

Service factories are the most flexible and versatile way to register a service. To register a factory, you use the registerFactory() method. It takes three parameters: the service key as the first parameter, the factory function as the second parameter, and an options object as the third parameter.

This factory function accepts the context object as an argument and should return the corresponding service. Dependencies of the constructed service can be obtained from the context object using the resolve(), resolveAll(), or resolveTuple() methods.

This factory function will be invoked when the service is requested from the container or as a dependency of another service in another factory function.

The lifecycle of the created service is regulated by the lifecycle option, which can have one of three values:

• "transient" (default) - for each request of this service, the factory function will be called, generally returning a new instance of the service.
• "singleton" - the factory function will be called once, after which the created instance of the service will be stored in the container. For all subsequent requests, the same instance of the service will be returned throughout the application's lifetime (assuming the service registration is not updated).
• "request" - operates similarly to "singleton", but only within the scope of a single root request. A root request is considered to be a call to one of the resolve(), resolveAll(), or resolveTuple() methods on the container instance. Thus, when resolving a service of one root request, all services that depend on the service with the "request" lifecycle will receive the same instance of it, but in the next root request, this instance will be different.

In addition to lifecycle, the options of the registerFactory() method also include name and replace, the meaning and action of which are identical to the similarly named options of the registerConstant() method.

// Register interface implementation as singleton
container.registerFactory(
  "HttpClient",
  () => new ConcreteHttpClient(),
  { lifecycle: "singleton" }
);

// Register interface implementation with dependencies
container.registerFactory(
  "BackendApiClient",
  (context) => new ConcreteBackendClient(context.resolve("HttpClient"))
);

// Register service factory, using constructor as key
container.registerFactory(TextEncoder, () => new TextEncoder());

Registering Class Configuration

In general, when only dependency injections through a class constructor are used, your class factories may look quite similar:

container.registerFactory(
  MyClass,
  (context) => new MyClass(context.resolve("Dep1"), context.resolve("Dep2"))
)

To free you from routine and make class factory registration more declarative, the registerClassConfig() method is designed. It takes the class constructor as the first argument, and as the second argument, it accepts a tuple of dependencies, the corresponding members of which will be used to extract the constructor dependencies in the respective order.

The third argument is an options object. In addition to options of registerFactory() method, there are one more option:

• circular - this should be set to true, when class has circular dependencies. See details below in corresponding section.

Please note that you can pass dependencies that are not directly extracted from the container by using a constant token. Typically, this applies to primitive data types that do not make much sense to store in the container.

You can use the constant() helper for convenience in creating constant tokens.

import { constant } from "@alxcube/di-container";

// Register different configurations with constant token
container.registerClassConfig(
  TextDecoder,
  [{ constant: "utf-8" }],
  { name: "utf8" }
);
container.registerClassConfig(
  TextDecoder,
  [constant("koi8-r")], // use `constant()` helper
  { name: "koi8" }
);

// Resolving
const utf8Decoder = container.resolve(TextDecoder, "utf8");
const koi8Decoder = container.resolve(TextDecoder, "koi8");

// Registering class config with container dependencies
container.registerClassConfig(
  PaymentsApiClient,
  [
    { service: "HttpClient", name: "payment" }, // Dependency on service with specific name
    "XmlParser", // Dependency on default service
    TextDecoder, // Dependency on class
  ]
);

You also can use classNames Map for binding constructors to their names. This helps to keep meaningful class names in error messages after your code gets minified.

import { classNames } from "@alxcube/di-container";

classNames.set(ConcreteHttpClient, "ConcreteHttpClient");

Registering Interface Implementation

Similarly, the implement() method works like the registerClassConfig() method, allowing you to declaratively bind an interface to its implementing class. The first argument of the method takes the string name of the interface, which is key of the service map. The second argument is the constructor of the class implementing this interface. The third argument is a tuple of class dependencies, just like in registerClassConfig(). The fourth argument is options, which are the same as in registerClassConfig().

// Register implementaion with no dependencies
container.implement("HttpClient", ConcreteHttpClient, []);

// Register implementation with dependencies
container.implement("BackendApiClient", ConcreteBackendClient, ["HttpClient"]);

Generating Array Resolvers

Some of your classes may depend on an array of homogeneous interfaces from the container. Typically, such a dependency is resolved using the resolveAll() method inside the service factory:

container.registerFactory(
  "HttpClientsPool",
  (context) => new ConcreteHttpClientsPool(
    context.resolveAll("HttpClient")
  )
);

To be able to leverage the benefits of declarative dependency specification in methods like registerClassConfig() or implement(), you can use the createArrayResolver() method.

First, add a separate type for the array of the selected service to your service map. For example, it might look like this:

interface AppServiceMap {
    HttpClient: HttpClient;
    "HttpClient[]": HttpClient[];
}

Then pass the keys of the single type and the corresponding array type to the createArrayResolver() method:

container.createArrayResolver("HttpClient", "HttpClient[]");

This will be equivalent to the following code:

container.registerFactory("HttpClient[]", (context) => context.resolveAll("HttpClient"));

Now you can declaratively use the dependency on the array:

container.implement("HttpClientPool", ConcreteHttpClientsPool, ["HttpClient[]"]);

The createArrayResolver() method also accepts options similar to the options of the registerFactory() method.

Child Containers

The createChild() method creates an empty child container. It is "empty" in the sense that it initially does not have its own service registrations, but all services registered in the parent container are also accessible in the child container.

When registering services in the child container, they override registrations with the same name from the parent container.

When removing a service from the child container, the existing registration from the parent container (if it exists) is not removed unless the cascade parameter of the unregister() method is set to true.

To obtain the parent container, you can use the getParent() method. It returns the parent container or undefined if the container has no parent.

It is important to note that when requesting a service from the child container, a process of merging all registrations across the container hierarchy occurs, which may lead to performance degradation when there are a large number of registrations.

// Register interface implementation as singleton
container.implement("HttpClient", ConcreteHttpClient, [], { lifecycle: "singleton" });

// Create child container
const childContainer = container.createChild();

// Resolve http client from child container
const httpClient1 = childContainer.resolve("HttpClient");

// Override interface implementation in child container. (No need to pass `true` as
// `replace` option value, since child container hasn't own registration of HttpClient
childContainer.implement(
  "HttpClient",
  AxiosHttpClient,
  ["AxiosInstanceFactory"],
  { lifecycle: "singleton" }
);

// Resolve new implementation of http client from child container
const httpClient2 = childContainer.resolve("HttpClient");

// Now there are different implementations in parent and child containers.
console.log(httpClient1 === httpClient2); // false

// Parent container keeps old registration
console.log(httpClient1 === container.resolve("HttpClient")); // true

Checking Service Existence

The has() method takes a service key and an optional service name as arguments and returns true if such a service is registered in the container, and false otherwise. If no name is provided, the method returns true if there is at least one registration for the service with that key. If a name is provided, it checks for the existence of a registration with that specific name.

The hasOwn() method works similarly, with the only difference being that, unlike the has() method, which checks for registrations in parent containers as well, the hasOwn() method only checks for service registration in the container on which it was called.

// assume that HttpClient is registered in parent container, and not registered in child
console.log(childContainer.has("HttpClient")); // true
console.log(childContainer.hasOwn("HttpClient")); // false
console.log(container.has("HttpClient")); // true
console.log(container.hasOwn("HttpClient")); // true

// Check with name
console.log(container.has("HttpClient", "default")); // true
console.log(container.has("HttpClient", "not-registered-name")); // false

Unregistering Service

To remove a service registration, the unregister() method is used. The only mandatory argument is the service key. The second argument is the service name. The third argument, cascade, indicates whether the service should also be removed from all parent containers.

If the service name is not specified (or is undefined), all service registrations with that key will be removed. If a name is provided, only the registration with the corresponding name will be removed.

// Unregister HttpClient from child container.
childContainer.unregister("HttpClient");

// Unregistering services that are not registered does nothing
childContainer.unregister("HttpClient");
childContainer.unregister("HttpClient", "default");

// Unregister service from whole container hierarchy
console.log(parentContainer.has("HttpClient")); // true
childContainer.unregister("HttpClient", undefined, true);
console.log(parentContainer.has("HttpClient")); // false

Circular dependencies

If your classes have circular dependencies, and for some reason you cannot refactor to eliminate them, there are 2 ways to register classes with circular dependencies.

Using circular() Helper

The first way is to wrap your service factory using the circular() helper:

import { circular } from "@alxcube/di-container";

class CircularA {
  constructor(private readonly circularB: CircularB) {}
}

class CircularB {
  constructor(private readonly circularA: CircularA) {}
}

container.registerFactory(
  CircularA,
  circular(
    (context) => new CircularA(
      context.resolve(CircularB)
    )
  )
);
container.registerFactory(
  CircularB,
  circular(
    (context) => new CircularB(
      context.resolve(CircularA)
    )
  )
);

This function will return a service factory that creates a JavaScript Proxy, replacing the requested class, and only when this object is first accessed, your factory will be called, which will create an instance of the class.

If you register classes with circular dependencies using the registerClassConfig() or implement() methods, set the circular option to true. Under the hood, this will wrap the generated service factory using the circular() helper.

Using Delayed Dependencies Injection

The second approach is delayed dependency injection. This method is less convenient and not as universal, but it does not use proxies, so it may be useful if proxies are not available in your application environment, although this is unlikely.

To implement this approach, your class dependencies must be injected through public properties or must have setter methods. Additionally, the "singleton" or "request" lifecycle is a mandatory requirement for such circular dependencies.

You can pass a callback to the delay() method of the context object in the service factory, in which you can resolve and set the necessary dependencies. This callback will be invoked after resolving the dependency stack of the current service.

class CircularA {
  constructor(public circularB: CircularB = undefined) {}
}

class CircularB {
  constructor(public circularA: CircularA = undefined) {}
}

container.registerFactory(
  CircularA,
  (context) => {
    // Create instance without dependency
    const instance = new CircularA();
    // Delay resolution and injection of dependency
    context.delay(() => instance.circularB = context.resolve(CircularB));
    // Return instance
    return instance;
  },
  {
    // Lifecycle must be either "request" or "singleton"
    lifecycle: "request"
  }
);

// Same for other circular dependency
container.registerFactory(
  CircularB,
  (context) => {
    const instance = new CircularB();
    context.delay(() => instance.circularA = context.resolve(CircularA));
    return instance;
  },
  { lifecycle: "request" }
);

Service Modules

To categorize registrations in the container and separate code, you can use service modules. These are simple objects with a single register() method, which takes the container object as its sole parameter.

To activate a module, pass it to the loadModule() method of the container.

// module.ts
import type { ServiceModule } from "@alcube/di-container";
import type { AppServiceMap } from "./AppServiceMap";
export const module: ServiceModule<AppServiceMap> = {
  register(container) {
    container.registerConstant("applicationKey", "some-app-key");
  }
}

// container.ts
import { Container } from "@alxcube/di-container";
import type { AppServiceMap } from "./AppServiceMap";
import { module } from "./module";

const container = new Container<AppServiceMap>();
container.loadModule(module);

Testing

The container also provides some methods that are useful for testing purposes.

Container Snapshots

Using the backup() method, you can create snapshots of the container's state, and with the restore() method, you can roll back the container's state to a previous snapshot. Snapshots work on a stack principle, and their number is unlimited. Typically, you would use the backup() and restore() methods, respectively, in the beforeEach() and afterEach() hooks of your testing framework.

Calling the backup() method without parameters creates a snapshot of the container on which it was called. However, if the optional parameter cascade is set to true, this method will also be called on all parent containers, causing them to create snapshots of their own state. The restore() method works similarly. Be careful when using cascading snapshots and remember to set the cascade parameter to true for the corresponding restore() calls, otherwise, you may encounter hard-to-track container state violations.

let httpClientSpy: HttpClientSpy;

beforeEach(() => {
  container.backup();
  httpClientSpy = new HttpClientSpy();
  container.registerConstant("HttpClient", httpClientSpy, { replace: true });
});

afterEach(() => {
  container.restore();
})

Creating Class Instances

The instantiate() method exists for conveniently creating instances of a class with dependency injection through the constructor, using the container. This method takes the class constructor as the first argument and a tuple of dependencies as the second argument, and returns an instance of the provided class. This is convenient for use in unit testing specific classes.

let backendClient: ConcreteBackendClient;

beforeEach(() => {
  backendClient = container.instantiate(ConcreteBackendClient, ["HttpClient"]);
})

Contextual Dependencies Resolving

To contextually resolve dependencies, you can use the methods isResolvingFor() and isDirectlyResolvingFor() of the context object inside service factories. Both methods take a service key and an optional service name as parameters.

The first method returns true if the current service (returned by the factory) is resolved as a dependency at any level for the corresponding service. This means, for example, that the current service can be a dependency of a dependency of the service whose key is passed to the method.

The second method is similar to the first one but checks if the current service is a direct dependency of the corresponding service.

If the name argument is not provided, only the service key is considered, and the name is ignored. To check if the current service is resolved specifically for the default registration of another service, pass "default" as the second argument explicitly.

You can also get the entire current dependency resolution stack by calling the getStack() method of the context object. The stack is an array of named service keys, where the first element is the service key requested from the container, and the last element is the key of the current service (in whose factory the check is performed).

Note that for this factory to work correctly, it must have a 'transient' lifecycle.

// Register class configs for implementations (this may be singletons)
container.registerClassConfig(LocalFileSystemDriver, [], { lifecycle: "singleton" });
container.registerClassConfig(CloudFileSystemDriver, ["HttpClient"]);

// Register contextual service factory (this must be transient)
container.registerFactory("FileSystem", (context) => {
  if (context.isResolvingFor("UserpicRepository")) {
    return context.resolve(LocalFileSystemDriver);
  }
  return context.resolve(CloudFileSystemDriver);
});

Service Resolution Error

When an error occurs during the resolution of a service, a ServiceResolutionError will be thrown.

This error class contains a stack property representing the service resolution stack. The stack is an array of named service keys, where the first element is the service key requested from the container, and the last element is the key of the service in whose factory the error occurred.

The cause property of the ServiceResolutionError object contains the caught value that caused the failure.

The message property contains a string that includes the string representation of the caught value, as well as the textual representation of the resolution stack in reverse order: the first line of the stack represents the key and name of the service in whose factory the error occurred.