Count Substrings That Differ by One Character LeetCode solution in JavaScript

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var countSubstrings = function (s, t) {
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  const count1 = countAllSubstr(s);
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  const count2 = countAllSubstr(t);
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  let res = 0;
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  for (const [substr1, freq1] of count1) {
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    for (const [substr2, freq2] of count2) {
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      if (differByOneChar(substr1, substr2)) {
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        res += freq1 * freq2;
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      }
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    }
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  }
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  return res;
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  function countAllSubstr(str) {
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    const n = str.length;
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    const count = new Map();
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    for (let i = 0; i < n; i++) {
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      let substr = "";
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      for (let j = i; j < n; j++) {
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        substr += str.charAt(j);
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        if (!count.has(substr)) count.set(substr, 0);
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        count.set(substr, count.get(substr) + 1);
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      }
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    }
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    return count;
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  }
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  function differByOneChar(str1, str2) {
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    if (str1.length != str2.length) return false;
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    const n = str1.length;
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    let missed = 0;
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    for (let i = 0; i < n; i++) {
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      const char1 = str1.charAt(i);
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      const char2 = str2.charAt(i);
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      if (char1 != char2) missed++;
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      if (missed > 1) return false;
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    }
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    return missed === 1;
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  }
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};