Should you use 'var' or 'let' in the global scope?

Question

Let's say I have this express application and want to add a global variable at the top

import express from 'express';

const app = express();

// var globalScopeVariable = 123;

app.get('/', (req, res) => {
  // home page
});

app.get('/create/:message', (req, res) => {
  //  add block
});

app.get('/add/:peerPort', (req, res) => {
  // add peer
});

Would it be good practice to use 'var' or 'let' in this scenario?

Answer 1

In your case (Node.js), neither var nor let make a global scope variable; both will create a module-scope variable (accessible inside this module, but not in other modules). This may be what you want (in which case, let is generally preferred these days, and var should be consigned to history); but in case it isn't, the only way to make a true global scope variable is by direct assignment:

global.globalScopeVariable = 123;

Answer 2

If your variable can be reassigned, then use let otherwise use const. You don't have to bother about var anymore.

You can always consider the following powerful master rules around variable declaration in modern JavaScript.

• Stop using var as soon as you can!
• Use const whenever you can!
• Use let only when you really have to!