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Dispatch by file JS: Polymorphism

Another interesting trick is dispatching by file name.

In some systems, it's common not to have one file with different keys for configuration, but different files belonging to different environments. For example:


Somewhere in the source code must be a piece of code that selects which file to load. The code below uses dispatching by key:

import fs from 'fs';

const configFileNamesByEnv = {
  development: 'database.development.json',
  production: 'database.production.json',
  test: 'database.test.json',

const filename = configFileNamesByEnv[env];
const raw = fs.readFileSync(filename);
const config = JSON.parse(raw);

It's not difficult to see that by having a startup environment name, it's possible to create a suitable file name. Let's do that:

const filename = `database.${env}.json`;
const raw = fs.readFileSync(filename);
const config = JSON.parse(raw);

The code is much shorter and no longer needs to be changed when expanding.

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