Encode and Decode TinyURL LeetCode solution in JavaScript

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let arr = new Array();
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let size = 0;
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var encode = function (longUrl) {
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  arr.push(longUrl);
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  let i = longUrl.indexOf("/", 11);
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  longUrl.slice(i);
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  longUrl += this.size;
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  arr.push(longUrl);
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  size += 2;
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  return longUrl;
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};
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var decode = function (shortUrl) {
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  let i = 0;
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  while (arr[i] !== shortUrl && i < size) {
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    i++;
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  }
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  return arr[i - 1];
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};