I would like add two point.

1. The while loop is not necessary, because we are asked for longest substring, so we do not have to waist CPU to shrink the string or array in the next task on more than 1 element. So if statement with just one iteration is enough
2. In JS you can make use of Set data structure, that is more elegant as HashMap.

const func = (_str, _k) => { let result = []; let windowStart = 0; let setUnique = new Set() // Set keeps only uniq elements

for (let windowEnd = 0; windowEnd < _str.length; windowEnd++) { const _arr = [..._str] // transform string to array result.push(_arr[windowEnd]) setUnique.add(_arr[windowEnd])

if (setUnique.size > _k) { result.shift(); setUnique.delete(_arr[windowStart]) windowStart += 1; } } return result.length; };