function zip(s){

  if(!s.length) return s;

  let za=[];
  let zv=[];

  for(
    let i=0,ind=-1,cur=false;
    i<s.length;
    i++
  ){
    if(cur!=s[i]){
      ind++;
      cur=s[i];

      za[ind]=s[i];
      zv[ind]=1;

      continue;
    }
    zv[ind]++;
  }

  let res='';
  for(let i=0;i<za.length;i++){
    res+=za[i];
    if(zv[i]>1)res+=zv[i];
  }

  return (res.length<s.length)?res:s;
}

console.log(
[
  ['test','test'],
  ['aaab','a3b'],
  ['aaabb','a3b2'],
  ['aabb','aabb'],
  ['abbb','ab3'],
  ['aaaa','a4'],
].map(i=> (zip(i[0])==i[1]))
)