LFU Cache LeetCode solution in JavaScript

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/**
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 * @param {number} capacity
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 */
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var LFUCache = function (capacity) {
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  this.capacity = capacity;
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  this.cache = [];
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};
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/**
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 * @param {number} key
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 * @return {number}
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 */
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LFUCache.prototype.get = function (key) {
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  let val = -1;
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  if (!this.capacity) return val;
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  const existIndex = this.cache.findIndex((item) => item.key === key);
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  if (existIndex > -1) {
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    const item = this.cache[existIndex];
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    val = item.value;
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    item.count++;
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    this.cache.splice(existIndex, 1);
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    this.cache.unshift(item);
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  }
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  return val;
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};
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/**
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 * @param {number} key
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 * @param {number} value
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 * @return {void}
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 */
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LFUCache.prototype.put = function (key, value) {
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  if (!this.capacity) return;
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  const existIndex = this.cache.findIndex((item) => item.key === key);
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  if (existIndex > -1) {
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    // new item already exists，rewrite the value and increase count
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    const existItem = this.cache[existIndex];
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    existItem.value = value;
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    existItem.count++;
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    this.cache.splice(existIndex, 1);
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    this.cache.unshift(existItem);
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  } else {
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    // new item doesn't exist
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    if (this.cache.length === this.capacity) {
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      // reach the capacity, need to clear LFU
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      let lfuIndex = 0;
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      let leastCount = this.cache[lfuIndex].count;
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      for (let i = 1; i < this.cache.length; i++) {
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        const item = this.cache[i];
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        if (item.count <= leastCount) {
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          leastCount = item.count;
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          lfuIndex = i;
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        }
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      }
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      this.cache.splice(lfuIndex, 1);
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      // after clear LFU, push the new item
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      this.cache.unshift({
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        key,
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        value,
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        count: 1,
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      });
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    } else {
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      // new item can be pushed
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      this.cache.unshift({
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        key,
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        value,
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        count: 1,
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      });
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    }
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  }
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};
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/**
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 * Your LFUCache object will be instantiated and called as such:
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 * var obj = new LFUCache(capacity)
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 * var param_1 = obj.get(key)
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 * obj.put(key,value)
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 */