test.js

function canCast(aPool, ...aCasts) {
    let pool = aPool.split('');
    let casts = toFlatArray(aCasts);

    casts = castLetters(pool, casts);

    if (!(stillHaveLettersMana(casts))) {  
      let requiredCastPoints = countCastPoints(casts);
      let availableCastPoints = countPoolPoints(pool);

      return (availableCastPoints >= requiredCastPoints) ?
         true :
            false
    } else {
        return false;
    }    
  }
  
  function stillHaveLettersMana(casts) {
    return casts.find(cast =>
        isColoredMana(cast)
    )
  }

  function toFlatArray(casts) {
    let arr = casts.map(cast =>
        cast.split('')
    )
    return [].concat.apply([], arr);
  }

  function castLetters(pool, casts) {
    return casts.filter ( letter => {
        if (isColoredMana(letter)) {
            return (castPool(pool, letter)) ? 
                    false :
                        true
        } else {
            return true
        }
    })
  }
  
  function castPool(pool, letter) {
    if (pool.indexOf(letter) > -1) {
      pool.splice(pool.indexOf(letter), 1)
      return true 
    } else {
      return false
    }
  }
  
  function isColoredMana(letter) {
    let coloredMana = ["B", "G", "R", "U", "W"]
    return coloredMana.indexOf(letter) > -1 ?
        true :
            false
  }
  
  function countCastPoints(casts) {
    return casts.reduce((first, second) =>
      parseInt(first) + parseInt(second)
    , 0)
  }
  
  function countPoolPoints(pool = [...pool]) {
    let filteredMana = 0;
    pool = pool.filter(item => {
      if (isColoredMana(item)) {
        filteredMana++;
        return false
      } else {
        return true
      }
    })
    
    let points = pool.reduce((first, second) => 
      parseInt(first) + parseInt(second)
    , filteredMana)
    return points;
    
  }