Count Prefixes of a Given String LeetCode solution in JavaScript

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var countPrefixes = function (words, s) {
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  let cont = 0;
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  for (i = 0; i < words.length; i++) {
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    for (j = 1; j <= s.length; j++) {
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      if (words[i] == s.slice(0, j)) {
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        cont++;
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      }
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    }
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  }
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  return cont;
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};