Is there a performance difference between 'let' and 'var' in JavaScript

Question

The difference between these two keywords in terms of scoping has already been thoroughly discussed here, but I was wondering if there is any kind of performance difference

Answer 1

Inside loops let is significantly slower see: https://jsperf.com/let-vs-var-loop

838,602 ±0.77% 61% slower

(function() {

  "use strict";
  var a=0;
  for(let i=0;i<100;i++) {
    a+=i;
  }
})();

vs.

2,136,387 ±1.09% fastest

(function() {

  "use strict";
  var a=0;
  for(var i=0;i<100;i++) {
    a+=i;
  }
})();

This is because when using let, for every loop iteration the variable is scoped. example:

for (let i = 0; i < 10 ; i++) {
    setTimeout(function() { console.log(i); }, 100 * i);
}

yields to

0,1,2,3,4,5,6,7,8,9

using var yields to

10,10,10,10,10,10,10,10,10,10

If you want to have the same result, but using var you have to use an IIFE:

for (var i = 0; i < 10; i++) {
  // capture the current state of 'i'
  // by invoking a function with its current value
  (function(i) {
    setTimeout(function() { console.log(i); }, 100 * i);
  })(i);
}

which on the other hand is significantly slower than using let.