How to print diamond shaped string of '*' javascript

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Javascript

I have been struggling to print a diamond shaped string with Javscript. I would like to know how I can insert spaces in the correct order. First, I was trying to print out a triangle and then I would turn it upside down and join them.
But I was stuck while trying to print the triangle.

What I have tried:

function diamond(arg) {
  if(arg === 1) {return "*"}
  var str = ""
  
   for(var j = arg; j > 1; j--) {
     str += Array(j).join(' ') + Array(j).join('*') + '\n'
    }
     
  
  return str
}
diamond(5) ->

    ****
   ***
  **
 *

But I would like it to print this

 **
****