Get Equal Substrings Within Budget LeetCode solution in JavaScript

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var equalSubstring = function (s, t, maxCost) {
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  let dp = [],
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    ans = 0;
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  for (let i = 0, j = 0, k = 0; i < s.length; i++) {
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    // overlay
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    k += dp[i] = abs(s[i], t[i]);
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    // non first
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    if (k > maxCost) {
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      (k -= dp[j]), j++;
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      continue;
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    }
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    // eligible
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    ans++;
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  }
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  return ans;
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  // get abs value
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  function abs(a, b) {
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    return Math.abs(a.charCodeAt(0) - b.charCodeAt(0));
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  }
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};