dependencyInjection.js

import readline from "readline-sync"

// Side effects

class EffectfulImplementation {
  getUserInput () {
    return readline.question("> ")
  }

  listTodoItems () {
    // Dummy implementation.
    return [
      { description: "Laundry", done: false },
      { description: "Groceries", done: true }
    ]
  }

  printTodoItems (items) {
    for (const item of items) {
      console.log(item)
    }
  }
}

// Logic (pure code)

function parseCommand(input) {
  // Dummy implementation.
  return {
    action: "list",
  }
}

function cli(impl) {
  const userInput = impl.getUserInput()
  const command = parseCommand(userInput)

  switch (command.action) {
    case "list":
      const items = impl.listTodoItems()
      impl.printTodoItems(items)
    // ...
  }
}

// Glue code

cli(new EffectfulImplementation());