finding index of item in nested array

If you're sure there's only one matching element in all your sub arrays, here's a little trick with flatMap.

let a = {
  sub: [
     {
      id: 1,
      s: [
      	{id: 5},
        {id: 1}
      ]
    },
    {
      id: 2,
      s: [
      	{id: 6},
        {id: 3}
      ]
    },
    {
      id: 3,
      s: [
      	{id: 9},
        {id: 2}
      ]
    }
  ]
}

console.log(a.sub.flatMap((a, i) => {
  const j = a.s.findIndex(z => z.id === 9);
  return j > -1 ? [i, j] : []
}));

This will return an array containing the index, i, in a.sub where a matching element is found followed by the index, j, in a.sub[i].s where the matching element was found.

Note flatMap is a relatively recent addition to the standard, so it may not work in older browsers. Be sure to use a polyfill or a transpiler like Babel, if this is a concern in your case.

Try this:

let a = {
  sub: [
     {
      id: 1,
      s: [
      	{id: 5},
        {id : 1}
      ]
    },
    {
      id: 2,
      s: [
      	{id: 6},
        {id: 3}
      ]
    },
    {
      id: 3,
      s: [
      	{id: 9},
        {id: 2}
      ]
    }
  ]
}

v = 9 

id1 = a.sub.findIndex(e => e.s.findIndex(ee => ee.id === v)!= -1)
id2 = a.sub[id1].s.findIndex(e => e.id === v )

console.log(id1) //index of the object
console.log(id2) //index within that object's s array

Modified answer of p.s.w.g, less likely to give you an eslint error.

let a = {
  sub: [
     {
      id: 1,
      s: [
        {id: 5},
        {id: 1}
      ]
    },
    {
      id: 2,
      s: [
        {id: 6},
        {id: 3}
      ]
    },
    {
      id: 3,
      s: [
        {id: 9},
        {id: 2}
      ]
    }
  ]
}

console.log(a.sub.flatMap((a, i) => {
  const j = a.s.findIndex(z => z['id'] === 9);
  return j > -1 ? [i, j] : []
}));