count all substring occurrences

kmp_search.js

const KMPSearch = (() => {
  const computeLPSArray = pat => {
    let len = 0;
    let i = 1;
    const lps = [0];
    while (i < pat.length) {
      if (pat[i] === pat[len]) {
        ++len;
        lps[i] = len;
        ++i;
      } else {
        if (len !== 0) {
          len = lps[len - 1];
        } else {
          lps[i] = len;
          ++i;
        }
      }
    }
    return lps;
  };
  return (pat, txt) => {
    const [m, n] = [pat.length, txt.length];
    let [i, j, res] = [0, 0, 0];
    const lps = computeLPSArray(pat);
    while (i < n) {
      if (pat[j] === txt[i]) {
        ++j;
        ++i;
      }
      if (j === m) {
        j = lps[j - 1];
        ++res;
      } else if (i < n && pat[j] !== txt[i]) {
        if (j !== 0) {
          j = lps[j - 1];
        } else {
          ++i;
        }
      }
    }
    return res;
  };
})();

const txt = "110100010101011";
const pat = "1010";
KMPSearch(pat, txt);