「組合わせ」を返す関数 - babu_babu

function mathematics (ary = [ ], n = ary.length) {
  if (1 === n) {
    return ary.map (a=> [a]);
  } else {
    return ary.flatMap ((a, i)=> mathematics (ary.slice (i), n -1).map (b=> [a,...b]))
  }
}

console.log (mathematics ([1,2,3],3));
/*
[1,1,1],
[1,1,2],
[1,1,3],
[1,2,2],
[1,2,3],
[1,3,3],
[2,2,2],
[2,2,3],
[3,3,3]
*/

やっぱり1行で済ます

const  mathematics=((f=(a=[],b=a.length)=>b==1?a.map(c=>[c]):a.flatMap((c,d)=>f(a.slice(d),b-1).map(d=>[c,...d])))=>f)();

で目的は

Javascript で、0000 から 9999 までの表を作りたい。 -HTML の <table- JavaScript | 教えて!goo

<!DOCTYPE html>
<title></title>
<meta charset="utf-8">
<style>
  td { text-align: right; width: 6em;}
</style>
<body>
<table border="1" id="T"></table>

<script>
const
  ary2tbody=(a,b=document.createElement('tbody'))=>a.reduce((b,a)=>(a.reduce((c,d)=>(c.insertCell().append(d||''),c),b.insertRow()),b),b),
  transpose=a=>a[0].map((_,i)=>a.map(b=>b[i])),
  mathematics=((f=(a=[],b=a.length)=>b==1?a.map(c=>[c]):a.flatMap((c,d)=>f(a.slice(d),b-1).map(d=>[c,...d])))=>f)(),
  sum=(a,b)=>a+b;


const A = [1, 2, 3, 4, 5, 6, 7, 8, 9];
const B = [[0], ...mathematics (A,1), ...mathematics (A,2), ...mathematics (A,3), ...mathematics (A,4)];
const C = [...Array(10)].map (()=>[]);

B.forEach (a=> C[a.reduce (sum, 0) % 10].push (a ? a.join``.padStart (4,0) : ''));
ary2tbody (transpose(C), T);

</script>