JSFiddle - React, Tailwind, and code Playground

JavaScript

function randomString(length) {
    var chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz'.split('');
    
    if (! length) {
        length = Math.floor(Math.random() * chars.length);
    }
    
    var str = '';
    for (var i = 0; i < length; i++) {
        str += chars[Math.floor(Math.random() * chars.length)];
    }
    return str;
}
var source = [];
for (var n = 1; n <= 100000; n++) {
    source.push(randomString());
}

console.time('test1');
var str1 = '';
for(var i = 0, l = source.length; i < l; i++) {
    str1 += source[i];
}
console.timeEnd('test1');

console.time('test2');
var str2 = source.join("");
console.timeEnd('test2');
console.log(str1 == str2);