Bad choice choosing "l" as a variable. Usually, the only single letter variable people use is "i" , for loop counting. Other than that, It is not recommended to use single letter variables since they are not descriptive. Also "l" looks like the number 1 , which can cause confusion when quickly scanning the code.

Here is another solution to the same problem, even though yours works....

function findLongestWord(wordsArr) {
  var longest = wordsArr[0];
  var candidate;

  for (var i = 1; i < wordsArr.length; i++) {
    candidate = wordsArr[i];

    if (candidate.length > longest.length) {
      longest = candidate;
    }
  }
  return longest;
}

var longest = findLongestWord(words);
console.log(longest);

Bad choice using "s" for the variable name. Why not just use "i" again ?