JSFiddle - React, Tailwind, and code Playground

JavaScript

function join(){
    var o="";
    for(var i=1;i<arguments.lengt;i++){
        if(o==="")
           o+=arguments[0];
        o+=arguments[i];
    }
}

console.log('t1', join(" ","hello","world") === "hello world");
console.log('t2', join(" ","hello","whole","universe") === "hello whole universe");
console.log('t3', join(",","hello","world") === "hello,world");
console.log('t4', join("!") === "");
console.log('t5', join() === "");