乘以数字的每个元素，直到长度为1

Question

如何将数字的每个元素乘以数字只有一个数字？

function persistence(number) {
  let a = number.toString();
  let b = 1;

  for (const ch of a) {
    b *= +ch
  }

  console.log(b) // 27
} 
// persistence(39)
// 39 --> 4 (because 3*9 = 27, 2*7 = 14, 1*4 = 4 and 4 has only one digit)

Answer 1

您可以使用递归调用persistence（）函数，直到结果具有length == 1

const result = persistence(47);
//--> returns 6 because: 4*7=28 => 2*8=16 => 1*6=6
console.log(result);

function persistence(num) {
  
  let a = num.toString();
  let b = 1;

  for (const ch of a) {
    b *= parseInt(ch);
  }    
  
  if (b.toString().length > 1)
    return persistence(b);  
    
  return b;
}