Why is let slower than var in a for loop in nodejs?

_{A note from the future: these historical performance differences are no longer accurate or relevant, as modern engines can optimize let semantics by using var semantics when there are no observable differences in behavior. When there are observable differences, using the correct semantics makes little difference in performance since the relevant code is already asynchronous in nature.}

Based on the difference between the mechanics of var vs. let, this discrepancy in runtime is due to the fact that var exists in the entire block scope of the anonymous function while let exists only within the loop and must be re-declared for each iteration.^{* see below} Here’s an example demonstrating this point:

(function() {
  for (var i = 0; i < 5; i++) {
    setTimeout(function() {
      console.log(`i: ${i} seconds`);
    }, i * 1000);
  }
  // 5, 5, 5, 5, 5


  for (let j = 0; j < 5; j++) {
    setTimeout(function() {
      console.log(`j: ${j} seconds`);
    }, 5000 + j * 1000);
  }
  // 0, 1, 2, 3, 4
}());

Notice that the i is shared across all iterations of the loop while let is not. Based on your benchmark, it appears that node.js just hasn’t optimized scoping rules for let since it’s much more recent and complicated than var is.

Elaboration

Here’s a little layman explanation of let in for loops, for those who don’t care to look into the admittedly dense specs, but are curious how let is re-declared for each iteration while still maintaining continuity.

But let can’t possibly be re-declared for each iteration, because if you change it inside the loop, it propagates to the next iteration!

First here’s an example that almost appears to validate this potential counter-argument:

(function() {
  for (let j = 0; j < 5; j++) {
    j++; // see how it skips 0, 2, and 4!?!?
    setTimeout(function() {
      console.log(`j: ${j} seconds`);
    }, j * 1000);
  }
}());

You are partially right, in that the changes respect the continuity of j. However, it is still re-declared for each iteration, as demonstrated by Babel:

"use strict";

(function () {
  var _loop = function _loop(_j) {
    _j++; // here's the change inside the new scope
    setTimeout(function () {
      console.log("j: " + _j + " seconds");
    }, _j * 1000);
    j = _j; // here's the change being propagated back to maintain continuity
  };

  for (var j = 0; j < 5; j++) {
    _loop(j);
  }
})();

Derek Ziemba brings up an interesting point:

Internet Explorer 14.14393 doesn’t seem to have these [performance] issues.

Unfortunately, Internet Explorer incorrectly implemented let syntax by essentially using the simpler var semantics, so comparing its performance is a moot point:

In Internet Explorer, let within a for loop initializer does not create a separate variable for each loop iteration as defined by ES2015. Instead, it behaves as though the loop were wrapped in a scoping block with the let immediately before the loop.

* This transpiled version on Babel’s REPL demonstrates what happens when you declare a let variable in a for loop. A new declarative environment is created to hold that variable (details here), and then for each loop iteration another declarative environment is created to hold a per-iteration copy of the variable; each iteration’s copy is initialized from the previous one’s value (details here), but they’re separate variables, as proven by the values output within each closure.