5.4 My Join created by smillaraaq - https://repl.it/HEVr/3

5.4 My Join.js

function myJoin(arr,sep) {
   let str = "";
   if (!sep) { sep = ","; }
   for (let i = 0 ; i < arr.length ; i ++) {
     if (arr[i] === undefined||arr[i] === null) {
         continue;
     }
     if (!!str.length) {
        str = str + sep;
     }
       
      str = str + arr[i];
   }
   return str;
}

function myJoin(arr,sep) {
   let str = "";
   if (!sep) {
       sep = ",";
   }
   for (let i = 0 ; i < arr.length ; i ++) {
       if (arr[i] === undefined||arr[i] === null) {
           continue;
       }else if (!str.length) {
           str = str + arr[i];
       } else {
           str = str + sep;
           str = str + arr[i];
       }
   }
   return str;
}

console.log(myJoin([null,undefined, 'hello', undefined, 'world', undefined], ' '));
// => 'hello world'
console.log(myJoin([2, "be", false]));
// => '2,be,false`