Expression Add Operators LeetCode solution in JavaScript

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var addOperators = function (num, target) {
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  let res = [];
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  let i = 0;
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  const helper = (idx, sum, prev, path) => {
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    if (idx >= num.length) {
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      if (sum === target) {
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        res.push(path);
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      }
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      return null;
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    }
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    for (let j = idx; j < num.length; j++) {
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      if (j !== idx && num[idx] === "0") break;
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      let n = Number(num.slice(idx, j + 1));
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      if (idx === 0) {
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        helper(j + 1, sum + n, sum + n, path + n);
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      } else {
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        helper(j + 1, sum + n, n, path + "+" + n);
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        helper(j + 1, sum - n, 0 - n, path + "-" + n);
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        helper(j + 1, sum - prev + prev * n, prev * n, path + "*" + n);
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      }
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    }
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  };
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  helper(i, 0, 0, "");
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  return res;
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};