Simple Express.js server that responds to basic HTTP methods.

const express = require('express');
const app = express();

// helper functions
const { getElementById, getIndexById, updateElement, seedElements, createElement } = require('./utils');

const animals = [];
seedElements(animals, 'animals');

const PORT = process.env.PORT || 4001;

// GET (read)
app.get('/animals', (req, res, next) => {
  res.send(animals);
});

// GET specific
app.get('/animals/:id', (req, res, next) => {
  const foundExpression = getElementById(req.params.id, animals);
  if (foundExpression) {
    res.send(foundExpression);
  } else {
    res.status(404).send();
  }
});

// PUT (update)
app.put('/animals/:id', (req, res, next) => {
  const expressionIndex = getIndexById(req.params.id, animals);
  if (expressionIndex !== -1) {
    updateElement(req.params.id, req.query, animals);
    res.send(animals[expressionIndex]);
  } else {
    res.status(404).send();
  }
});

// POST (create)
app.post('/animals', (req, res, next) => {
  const receivedExpression = createElement('animals', req.query);
  if (receivedExpression) {
    animals.push(receivedExpression);
    res.status(201).send(receivedExpression);
  } else {
    res.status(400).send();
  }
});

// DELETE
app.delete('/animals/:id', (req, res, next) => {
  const expressionIndex = getIndexById(req.params.id, animals);
  if (expressionIndex !== -1) {
    animals.splice(expressionIndex, 1);
    res.status(204).send();
  } else {
    res.status(404).send();
  }
});

// server listen
app.listen(PORT, () => {
  console.log(`Listening on port ${PORT}`);
});

Grouping routes into routers and then refactoring code to address this.

const expressionsRouter = express.Router();

app.use("/expressions", expressionsRouter);

// Get all expressions
// NOTICE that the path here is simply "/" since its specific endpoint is already defined at the router level.
expressionsRouter.get('/', (req, res, next) => {
  res.send(expressions);
});