JSFiddle - React, Tailwind, and code Playground

JavaScript

var count = 100000000, a = 0, start = new Date(), s="";

    for (var i=0; i<count; i++) {
      a = a + 1
    }
s+= "Declaring variable (i) in for loop expression takes: " + (new Date() - start) + 'milliseconds \n \n';
    start = new Date();

    var j;
    for (j=0; j<count; j++) {
      a = a + 1;
    }

    s+= "Declaring variable (j) outside for loop expression takes: " + (new Date() - start) + 'milliseconds \n \n';
    start = new Date();

    var j;
    for (j=0; j<count; j++) {
        var x;
        x = x + 1;
    }

    s+= "Declaring variable (x) inside for loop takes: " + (new Date() - start) + 'milliseconds \n \n';
    start = new Date();
    alert(s);