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

Issue

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 between the two, and if so, is it negligible, or at what point would it become significant?

Solution

After testing this on http://jsperf.com, I got the following results: jsperf has been down for a while; see the replacing code below.

To check this, I’ll use the following performance test based on this answer, which led me to write this function:

/**
 * Finds the performance for a given function
 * function fn the function to be executed
 * int n the amount of times to repeat
 * return array [time for n iterations, average execution frequency (executions per second)]
 */
function getPerf(fn, n) {
  var t0, t1;
  t0 = performance.now();
  for (var i = 0; i < n; i++) {
    fn(i)
  }
  t1 = performance.now();
  return [parseFloat((t1 - t0).toFixed(3)), parseFloat((repeat * 1000 / (t1 - t0)).toFixed(3))];
}

var repeat = 100000000;
var msg = '';

//-------inside a scope------------
var letperf1 = getPerf(function(i) {
  if (true) {
    let a = i;
  }
}, repeat);
msg += '<code>let</code> inside an if() takes ' + letperf1[0] + ' ms for ' + repeat + ' iterations (' + letperf1[1] + ' per sec).<br>'

var varperf1 = getPerf(function(i) {
  if (true) {
    var a = i;
  }
}, repeat);
msg += '<code>var</code> inside an if() takes ' + varperf1[0] + ' ms for ' + repeat + ' iterations (' + varperf1[1] + ' per sec).<br>'

//-------outside a scope-----------

var letperf2 = getPerf(function(i) {
  if (true) {}
  let a = i;
}, repeat);
msg += '<code>let</code> outside an if() takes ' + letperf2[0] + ' ms for ' + repeat + ' iterations (' + letperf2[1] + ' per sec).<br>'

var varperf2 = getPerf(function(i) {
  if (true) {}
  var a = i;
}, repeat);
msg += '<code>var</code> outside an if() takes ' + varperf1[0] + ' ms for ' + repeat + ' iterations (' + varperf1[1] + ' per sec).<br>'

document.getElementById('out').innerHTML = msg
<output id="out" style="font-family: monospace;white-space: pre-wrap;"></output>

After testing this in Chrome and Firefox, this shows that let is faster than var, but only when inside a different scope than the main scope of a function. In the main scope, var and let are roughly identical in performance. In IE11 and MS Edge, let and var are roughly equal in performance in both cases.

Press the big blue button to see for yourself in your favourite browser.

Currently let has support from only newer browsers, but older browsers are still being used relatively much, which would be a reason to generally not use it yet. If you want to use it somewhere where older browsers wouldn’t function otherwise, there should be no problem with it.

Edit: revamped answer since jsperf is not working (see revision history for old version).

Answered By – Joeytje50

Answer Checked By – Dawn Plyler (BugsFixing Volunteer)

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