JavaScript LookupList

LookupList.js

/*

use like so: 

const lookupList = new LookupList({ a: 1 }, { b: 2 }, { c: 3 })
console.log(lookupList.valueByKey('a'))  // 1
console.log(lookupList.keyByValue(1))  // "a"
console.log(lookupList.keysByValue(1))  // ["a"]
console.log(lookupList.valueByIndex(0))  // 1
console.log(lookupList.keyByIndex(0))  // "a"
console.log(lookupList.indexByKey('a'))  // 0

*/

class LookupList {

  constructor() {
    this._hashKey = {}
    this._hashVal = {}
    this._list = []
    this.length = 0;

    const objList = [].slice.call(arguments)

    for (let listObj of objList)
      this._push(listObj)
  }

  _push(listObj) {
    if (typeof listObj !== 'object')
      throw 'invalid argument in constructor'

    let keys = Object.keys(listObj)
    if (keys.length !== 1)
      throw 'invalid argument in constructor'

    let key = keys[0]
    let val = listObj[key];
    let obj = { key, val, index: this._list.length }

    if (this._hashKey[key])
      throw `duplicate key found: ${key}`

    this._hashKey[key] = obj

    if (this._hashVal[val])
      this._hashVal[val].keys.push(key)
    else
      this._hashVal[val] = { keys: [key] }

    this._list.push(obj)
    this.length = this._list.length
  }

  valueByKey(key) {
    if (!this._hashKey[key]) return undefined
    return this._hashKey[key].val
  }

  keyByValue(val) {
    if (!this._hashVal[val]) return undefined
    return this._hashVal[val].keys[0]
  }

  keysByValue(val) {
    if (!this._hashVal[val]) return undefined
    return this._hashVal[val].keys
  }

  valueByIndex(index) {
    if (!this._list[index]) return undefined
    return this._list[index].val
  }

  keyByIndex(index) {
    if (!this._list[index]) return undefined
    return this._list[index].key
  }
  
  indexByKey(key) {
    if (!this._hashKey[key]) return -1
    return this._hashKey[key].index
  }
}