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Dispatch by file JS: Polymorphism

Another interesting trick is dispatching by file name.

In some systems, it's common to have different files belonging to separate environments for configuration rather than one file. For example:


Somewhere in the source code must be a code block 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 is not difficult to see that using a startup environment name. So we can create a suitable file name. Let us 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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