Check if Word Equals Summation of Two Words LeetCode solution in JavaScript

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var isSumEqual = function (firstWord, secondWord, targetWord) {
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  let obj = {
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    a: "0",
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    b: "1",
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    c: "2",
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    d: "3",
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    e: "4",
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    f: "5",
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    g: "6",
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    h: "7",
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    i: "8",
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    j: "9",
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  };
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  let first = "",
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    second = "",
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    target = "";
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  for (let char of firstWord) {
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    first += obj[char];
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  }
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  for (let char of secondWord) {
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    second += obj[char];
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  }
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  for (let char of targetWord) {
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    target += obj[char];
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  }
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  return parseInt(first) + parseInt(second) === parseInt(target);
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};