code: function LRU(capacity, getSize) {
    if (!(this instanceof LRU))
      return new LRU(capacity, getSize);
  
    this.map = new Map();
    this.size = 0;
    this.items = 0;
    this.head = null;
    this.tail = null;
    this.pending = null;
  
    assert(typeof capacity === 'number', 'Capacity must be a number.');
    assert(capacity >= 0, 'Capacity cannot be negative.');
    assert(!getSize || typeof getSize === 'function', 'Bad size callback.');
  
    this.capacity = capacity;
    this.getSize = getSize;
  }
docs: An LRU cache, used for caching {@link ChainEntry}s.
  @alias module:utils.LRU
  @constructor
  @param {Number} capacity
  @param {Function?} getSize