Inject mock data or an module into Node require

example.js

require('./require-inject')
  .inject('@mock/direct.json', { bla: 1 })
  .inject('@mock/file.json', require('./mock/file.json'))
  .inject('module-name', require('./some/module'));

const data1 = require('@mock/direct.json');
const data2 = require('@mock/file.json');
const mod1 = require('module-name');

require-inject.js

const path = require('path');
const m = require('module');

const root = path.join(path.resolve('.'), 'node_modules');

function get(res) {
  const id = resolve(res);
  const mod = require.cache[id];
  return {
    id: id,
    exists: mod !== undefined,
    mock: mod ? mod.mock : false
  };
}

// store real resolve function and add a fake resolver
const real_resolveFilename = m._resolveFilename;
m._resolveFilename = (request, parent) => { 
  // check if it's a 'fake' and return the id
  // this is for getting non-existent files
  const mod = get(request);
  if (mod.mock) {
    return mod.id;
  }
  
  // if it's not a 'fake' one, just use the defaut resolver
  return real_resolveFilename(request, parent);
};

// this could be better, but for normal usage it's enougth
function resolve(file) {
  // check if it's a module name (not absolute or relative)
  if (!path.isAbsolute(file) && /^[a-z]+/i.test(file)) {
    // return a 'fake' path using the default node_modules root
    return path.join(root, file, 'index.js');
  }
  return path.resolve(file);
}

const Injector = {
  inject: (name, exp) => {
    // check if the module already registered or not
    const mod = get(name);
    if (!mod.exists) {
      const res = mod.id;
      require.cache[res] = {
        id: res,
        filename: res,
        loaded: true,
        mock: true,
        exports: exp
      };
    }

    // return the injector for composable injects
    return Injector;
  },
  
  // yes we could add a method for removing the injectables
  // but, that's easy and this is just an example on it could be done
}

module.exports = Injector;