h.js

class DumbMap {
  constructor() {
    this.list = []
  }

  get(x) {
    let result

    this.list.forEach(pairs => {
      if (pairs[0] === x) {
        result = pairs[1]
      }
    })

    return result
  }

  set(x, y) {
    this.list.push([x, y])
  }
}

let m = new DumbMap()

m.set('a', 1)
console.log(m.get('a')) // 1
console.log(m.get('I_DONT_EXIST')) // undefined