Javascript strings are immutable

This means they never change. Most of the time it doesn’t matter.

function CONCATappendSlow(N){
   console.time();
   var H ='';
   for (var i=0; i < N; i++){
      H+='The quick' + " ";
   }
   console.timeEnd();
   return H;
}

And this is a faster way of doing it.

function CONCATappendFast(N){
   console.time();
   var H = new Array(N);
   for (var i=0; i < N; i++){
      H[i] ='The quick';
   }
   console.timeEnd();
   console.time();
   var Result = H.join(' ');
   console.timeEnd();
   return Result;
}