constructor-injection

Install

npm install constructor-injection

Example Usage

Constructor Injection

import { injectConstructor } from "constructor-injection";
 
@Injectable() // from Inversify or Angular for example
class ClassWithParameters {
    constructor(
        public readonly paramOne: string,
        public readonly paramTwo: number,
        public readonly paramThree: boolean) {
    }
}
 
const resolveParameter: ParameterProvider = (passedParameter: any, reflectMetadata: any, index: number) => {
    // Logic to provide correct parameter value
    // For example if inversify is your DI library:
    return passedParameter != null ? passedParameter : inversifyContainer.get(reflectMetadata);
};
 
const injectedConstructor = injectConstructor(ClassWithParameters, resolveParameter);
 
const instance = new injectedConstructor();

Function Injection

A similar approach can be used to provide values for function parameters. Unfortunately the metadata for function parameters isn't currently available so we need to provide the metadata for the function parameters ourselves.

import { injectFunction } from "constructor-injection";
 
function functionWithParameters(
    paramOne: string,
    paramTwo: ClassWithNoParams,
    paramThree: ClassWithParameters): string {
    return "return value";
}
 
const resolveParameter: ParameterProvider = (passedParameter: any, reflectMetadata: any, index: number) => {
    // Logic to provide correct parameter value
    // For example if inversify is your DI library:
    return passedParameter != null ? passedParameter : inversifyContainer.get(reflectMetadata);
};
 
const injectedFunction = injectFunction( // typed as (paramOne?: string, paramTwo?: ClassWithNoParams, paramThree?: ClassWithParameters) => string
    functionWithParameters, 
    [ undefined, ClassWithNoParams, ClassWithParameters ], // provide types to inject 
    resolveParameter);
 
injectedFunctions("stringValueThatCantBeInjected"); // call function

Setup

This package uses Reflect Metadata to inspect the constructor parameters of your class. To use this your tsconfig.json must contain:

{
    "compilerOptions": {
        "experimentalDecorators": true,    
        "emitDecoratorMetadata": true      
    }
}

and the class that you are trying to construct MUST have a decorator to inform typescript that metadata should be saved:

@Injectable()
class ClassWithParameters {
    constructor(
        public readonly paramOne: string,
        public readonly paramTwo: number,
        public readonly paramThree: boolean) {
    }
}

you must also install and import reflect-metadata somwhere in your app - preferably as the first import.

npm install reflect-metadata

import "reflect-metadata";

Minimum Typescript version: 3.5