Simple function injection

inject.js

export default function inject() {
  const args = [...arguments];
  const toCall = args[0];
  const toInject = args.slice(1, args.length);
  const shouldInject = inject.should === false ? false : true;

  if (shouldInject) {
    return function () {
      const combinedArgs = [...toInject, ...arguments];

      return toCall.apply(this, combinedArgs);
    };
  }
  else {
    return toCall;
  }
}

inject.test.js

import { expect } from 'chai';
import inject from '../inject';

const { describe, it } = global;

describe('inject utility', () => {
  const adder = (a, b, c) => {
    return a + b + c;
  };

  const caller = (a, b) => {
    return a(b);
  };

  it('can inject a dependency without changing the parameters', () => {
    const useDeps = inject(adder, 1);

    expect(
      useDeps(2, 3)
    ).to.be.equal(6);
  });

  it('can inject functions', () => {
    const useDeps = inject(caller, (a) => { return a * 2; });

    expect(
      useDeps(4)
    ).to.be.equal(8);
  });

  it('can be disabled', () => {
    inject.should = false;

    const useDeps = inject(caller, (a) => { return a * 2; });

    expect(
      useDeps(a => { return a * 4; }, 4)
    ).to.be.equal(16);

    inject.should = true;
  });
});