const vs let when calling require

const can be normally used when you don't want your program

1. to assign anything to the variable

   "use strict";
   const a = 1;
   a = 2;
   

   will produce TypeError: Assignment to constant variable..

2. to use the variable without explicitly initializing.

   "use strict";
   const a;
   

   will produce SyntaxError: Unexpected token ;

Simply put, I would say,

• use const whenever you want some variables not to be modified

• use let if you want the exact opposite of const

• use var, if you want to be compatible with ES5 implementations or if you want module/function level scope.

Use let only when you need block level scoping, otherwise using let or var would not make any difference.

Performance test const vs let usage on require for Node.js 6.10:

require('do-you-even-bench')([
  { name: 'test 1', fn: function() { 
    const path = require('path');
    } 
  },
  { name: 'test 2', fn: function() { 
    let path = require('path');
    } 
  }
]);

test 1 .... 2,547,746.72 op/s
test 2 .... 2,570,044.33 op/s

I have the same feeling that you're describing. A big percentage of declared variables in my code tend to be constant, even objects and arrays. You can declare constant objects and arrays and still be able to modify them:

const arr = [];
arr.push(1);
arr;
// [ 1 ]

const obj = {};
obj.a = 1;
obj;
// { a: 1 }

AFAIK ES6 modules do not require variable declarations when importing, and I suspect that io.js will move to ES6 modules in a near future.

I think this is a personal choice. I'd always use const when requiring modules and for module local variables (foo in your example). For the rest of variables, use const appropriately, but never go mad and use const everywhere. I don't know the performance between let and const so I cannot tell if it's better to use const whenever possible.