Letter Combinations of a Phone Number LeetCode solution in JavaScript

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var letterCombinations = function (digits) {
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  if (!digits) return [];
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  let res = [];
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  const alpha = {
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    2: "abc",
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    3: "def",
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    4: "ghi",
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    5: "jkl",
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    6: "mno",
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    7: "pqrs",
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    8: "tuv",
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    9: "wxyz",
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  };
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  const dfs = (i, digits, temp) => {
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    if (i === digits.length) {
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      res.push(temp.join(""));
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      return;
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    }
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    let chars = alpha[digits[i]];
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    for (let ele of chars) {
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      temp.push(ele);
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      dfs(i + 1, digits, temp);
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      temp.pop();
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    }
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  };
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  dfs(0, digits, []);
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  return res;
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};