Reversing words in a string JS without using built in function

Without using the split reverse and join functions, how would one do such a thing?

The Problem Given: Reverse the words in a string Sample Input: "Hello World" Sample Output: "World Hello"

<script>
  var newString = ""; 
  var theString = prompt("Enter a Phrase that you would like to reverse (Ex. Hello world)"); 

  newString = theString.split(" ").reverse().join(" ")


  document.write(newString);
</script>

Arrays can be used like stacks out of the box. And stacks are LIFO, which is what you need.

function reverseWords(str) {
    var word, words, reverse;

    words = str.match(/(?:\w+)/g);
    reverse = '';
    while(word = words.pop()) {
      reverse += word + ' ';
    }

    return reverse.trim();
}

reverseWords('hello world');

Or use the call stack as your stack:

function reverseWords(str) {
  var result = '';
  (function readWord(i = 0) {
    var word = '';
    if(i > str.length) {
        return '';
    }    
    while(str[i] !== ' ' && i < str.length) {
      word += str[i];
      i++;
    }    
    readWord(++i); // Skip over delimiter.
    result += word + ' ';    
  }());
  return result.trim();
}

reverseWords('hello world');