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const foo = function(string) {
  if (!string) return
   // If string is empty, return 0
    if (!string) return 0
  
    // Holds our count of highest substring length
    let counter = 0;  
  

    // Loop  will add elements here
    let tmpArray = []
  
    const arr = string.split("")
    arr.forEach((e, i) => {
        // If this character already exists in tmpArray...
        if (tmpArray.indexOf(e) != -1) {
            // Update counter if tmpArray's length if higher
            if (tmpArray.length > counter) {
                counter = tmpArray.length
            }
            
            // Clear tmpArray with the current element in it
            tmpArray = [e]
            return
        }
        
        // Else, if tmpArray does not have this character in it, add it
        tmpArray.push(e)
        console.log(tmpArray)
    })
    
    if (tmpArray.length > counter) {
      counter = tmpArray.length
    }
  
  return counter
};

const r = foo("dvdf")
console.log(r)

/*
[a]
[ab]
[abc]
theres an a.set counter to tmp.length if temp.length > counter
clear tmp array
*/