minimal dependency injection example in JavaScript

dependency-injection.js

// minimal working dependency injection
// the dependency injection works with a property based injection

const clazzMap = Symbol('clazzMap');
globalThis[clazzMap] = new Map(); // <clazz, instance>
const clazzes = Symbol('clazzes')
globalThis[clazzes] = [];

function register(clazz) {
    globalThis[clazzes].push(clazz);
}

function inject(clazz) {
    if (globalThis[clazzMap].has(clazz)) {
        return globalThis[clazzMap].get(clazz);
    }
    if (globalThis[clazzes].includes(clazz)) {
        const instance = new clazz();
        globalThis[clazzMap].set(clazz, instance);
        return instance;
    }
    throw new Error(`no class registered for class ${clazz.name}`);
}

// Example of usage with JavaScript classes

class A {
    name = 'barA';
}

class B {
    name = 'barB';
    c = inject(C);
}

class C {
    name = 'barC';
    a = inject(A);
}

// order of registering does not matter
register(C);
register(A);

// first injection can only happen after all needed classes are registered
const a = inject(A);

// you can register new classes delayed as long as previous injects to not rely on it
register(B);
const b = inject(B);