'let' vs 'var' performance. just run the code below to see the magic

LetVsVar.js

/**
 * 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)]
 */
const getPerf = (fn, n)  =>{
  let start = new Date().getTime()

  for (let i = 0; i < n; i++) fn(i)

  let end = new Date().getTime()

  return [parseFloat((end - start).toFixed(3)), parseFloat((repeat * 1000 / (end - start)).toFixed(3))]
}

let repeat = 100000000

//-------inside a scope------------
let letperf1 = getPerf(function(i) {
  if (true) {
    let a = i
  }
}, repeat)
console.log(`'let' inside an if() takes ${letperf1[0]} ms for ${repeat} iterations (${letperf1[1]} per sec).`)

let varperf1 = getPerf(function(i) {
  if (true) {
    var a = i
  }
}, repeat)
console.log(`'var' inside an if() takes ${varperf1[0]} ms for ${repeat} iterations (${varperf1[1]} per sec).`)

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

let letperf2 = getPerf(function(i) {
  if (true) {}
  let a = i
}, repeat)
console.log(`'let' outside an if() takes ${letperf2[0]} ms for ${repeat} iterations (${letperf2[1]} per sec).`)

let varperf2 = getPerf(function(i) {
  if (true) {}
  var a = i;
}, repeat);
console.log(`'var' outside an if() takes ${varperf2[0]} ms for ${repeat} iterations (${varperf2[1]} per sec).`)

This shows that 'let' is faster than 'var', but only when inside a different scope than the main scope of a function