Just use ternary operator to determine rendering last dash

function arr(a){
  let sum= "";
  for(let i=0; i<a.length; i++){
    i === a.length-1 ? sum+=a[i] : sum+= a[i]+"-";
  }
  console.log(sum);
}
arr(["Hello","World","!"]);

I think you only need to add one condition for the last item. For example

function arr(a){
  let sum= "";
  for(let i=0; i<a.length; i++){
      sum+= a[i] + ((i<a.length-1) ? "-" : "");
  }
  return sum
}
const result = arr(["Hello","World","!"]);

console.log(result)

Or it's neater with array reduce

const join = (arr, separator) => {
  return arr.reduce((prev, current)=> prev + separator +  current)
}

const rs =join(["Hello", "World", "!"], "-")
console.log(rs)

Same result

"Hello-World-!" 

you can do this as follow

function arr(a,glue='') {
  let sum = "";
  let i=0;
  for (i = 0; i < a.length-1; i++) {
    sum += a[i] + glue;
  }
  sum+=a[i];
 return sum;
}

console.log(arr(["Hello", "World", "!"],"-"));