JavaScript - 基礎面函數 - 0 ~ n 之間的數字之和

求 0 - n 之間的數字之和,

需要把 步驟 寫出來

輸入: 5 , 輸出: 0+1+2+3+4+5 = 15

輸入: 0 , 輸出: 0=0

輸入:-1 , 輸出: -1<0

var SequenceSum = (function() {
  function SequenceSum() {}

  SequenceSum.showSequence = function(count) {
    let result = 0
    let str = ''
    if(count < 0) return count + '<0'
    if(count == 0) return count + '=0'
    for(let i=0; i<=count; i++){
      result += i
      if(i == count) continue
      str += i + '+'
    }
    return str += count + ' = ' + result
  };

  return SequenceSum;