js一个数组各项组合的算法

Question

let arr = [1, 2, 3, 4];
(arr的长度不固定)
求 [1, 2, 3, 4, 12, 13, 14, 23, 24, 34]

Answer 1

var newArr = [];
for(var i=0;i<arr.length;i++){
  newArr.push(arr[i]);
  for(var x=i+1;x<arr.length;x++){
    newArr.push(arr[i]+''+arr[x]);
  }
}

Answer 2

var arr = [1, 2, 3, 4];
var i = [...arr];
i.forEach(value => {
  arr.shift();
  arr.forEach(v => {
    if (value != v) {
      i.push(Number('' + value + v));
    }
  })
}) //i [1, 2, 3, 4, 12, 13, 14, 23, 24, 34]