JS lambda default return value

Question

I am a bit confused with the default return value of lambdas in node.js. I found this link "Arrow Functions" :

Arrow functions can have either a "concise body" or the usual "block body".

In a concise body, only an expression is specified, which becomes the implicit return value. In a block body, you must use an explicit return statement.

var func = x => x * x;                  
// concise body syntax, implied "return"

var func = (x, y) => { return x + y; }; 
// with block body, explicit "return" needed

Which makes it pretty clear, but then I came upon this piece of Express code which I tested that returns the last statement by default without requiring to use "return":

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

app.use('/api/posts', (req, res, next) => {
  const posts = [
    {
      id: 'sdfj234j654j',
      title: 'First server-side post',
      content: 'This is comming from the server'
    },
    {
      id: '9054jk4ju59u90o',
      title: 'Second server-side post',
      content: 'This is comming from the server!'
    }
  ];
  // this is returned by default and does not require "return "
  res.status(200).json({
    message: 'Posts fetched succesfully!',
    posts: posts
  });
});

So which one is it do I need to use the return statement on lambdas when I use the block quotes to define them or not ? or is there an exception case which I am unaware of?

Answer 1

The arrow function in your example does not return anything. However it writes to the response by calling .json({ /*...*/}), therefore it kind of "returns" the json to the client.

A simplified example:

  setTimeout(() => {
    console.log("test");
  }, 1);

The above code outputs something to the console although nothing gets returned from the arrow function.