Additive Number LeetCode solution in JavaScript

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const getNum = (str, i, j) => {
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  str = str.slice(i, j);
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  if (str[0] == "0" && str.length > 1) return -1000;
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  return Number(str);
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};
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var isAdditiveNumber = function (num) {
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  // i = 3 say and make theory and proof that
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  const len = num.length;
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  for (let b = 2; b < len; b++) {
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    for (let i = 0; i < b - 1; i++) {
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      for (let j = i + 1; j < b; j++) {
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        let v1 = getNum(num, 0, i + 1);
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        let v2 = getNum(num, i + 1, j + 1);
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        let v3 = getNum(num, j + 1, b + 1);
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        if (v1 + v2 == v3) {
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          // test hypothesis;
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          // from b start checking if string persist behaviour
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          let p = num.slice(0, b + 1);
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          while (p.length <= len) {
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            let sum = v2 + v3;
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            p += sum;
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            v2 = v3;
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            v3 = sum;
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          }
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          if (p.slice(0, len) == num) {
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            return true;
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          }
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        }
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      }
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    }
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  }
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  return false;
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};