const arr=['plus1','plus3']

const obj = {
  'plus1':20,
  'plus2':30,
  'plus3':40 
}

let newArr = arr.map(function(val) {
  return this[val]
},obj)

console.log(newArr) // [20,40]


const a="-sub"

let newArr2 = arr.map(function(val) {
  return val + this
},a)

console.log(newArr2) // ["plus1-sub","plus3-sub"]