See: https://sampleprograms.io/projects/baklava/javascript/

My attempt at backlava in JS.

Fetaures and difference to other implementation:

• memoise the spaces and stars
• use of substr to print the number of spaces and stars
• use of a string builder instead of string concatenation
• the algorithm is contained within backlava()
• backlava() returns a string rather than printing directly to the console

function backlava(x) {
  let stars = '*'.repeat(x * 2)
  let spaces = ' '.repeat(x)
  let builder = []

  for (i = 1; i < x; i++) {
      builder.push(`${spaces.substr(i)} ${stars.substr((-i * 2) + 1)}`)
  }

  for (var i = x; i > 0; i--) {
    builder.push(`${spaces.substr(i)} ${stars.substr((-i * 2) + 1)}`)
  }

  let backlavaStr = builder.join("\n")

  return backlavaStr
}

console.log(backlava(20))

Output:

                    *
                   ***
                  *****
                 *******
                *********
               ***********
              *************
             ***************
            *****************
           *******************
          *********************
         ***********************
        *************************
       ***************************
      *****************************
     *******************************
    *********************************
   ***********************************
  *************************************
 ***************************************
  *************************************
   ***********************************
    *********************************
     *******************************
      *****************************
       ***************************
        *************************
         ***********************
          *********************
           *******************
            *****************
             ***************
              *************
               ***********
                *********
                 *******
                  *****
                   ***
                    *