kmp

<script>
l=console.log
target='abcab'
str='abcaabcab'

function weight(target){
  let [str,len,weight]=['','',[0]]
  for(i=2;i<=target.length;i++){
    str=target.slice(0,i)
    len=str.length
    for(i=1;i<len;i++){
      if(str.slice(0,len-i)==str.slice(i)){
        weight.push(len-i)
        //break
      }
    }
    weight.length<i ? weight.push(0) : ''
  }
  return weight
}

function kmp(str,target){
  weight=weight(target)
  k=0
  index=0
  for(i=0,j=0;i<str.length;i++,j++){
    if(target[j]==str[i]){
      k++
    }else{
      move=k-weight[j-1]
      i=index+move-1
      index+=move
      k=0
      j=-1
    }
    if(k==target.length){
      return i
    }      
  }  
}

l(kmp(str,target))

</script>