Skip Undefined Error if an object key is missing in Javascript

I would like to know as to how can we skip Undefined error being throw if a key is missing.

[
  {
    "id": "foo1",
    "name": "Test1",
    "description": "random test1",
    "languages": [
      "en"
    ],
    "subThemes": [
      {
        "id": "1",
        "name": "Bike"
      },
      {
        "id": "2",
        "name": "Testy",
        "description": "Testing this thing1"
      }
    ]
  },
  {
    "id": "foo2",
    "name": "Test",
    "description": "random test2",
    "languages": [
      "en"
    ],
    "subThemes": [
      {
        "id": "3",
        "name": "Bike2"
      },
      {
        "id": "4",
        "name": "Testy2",
        "description": "Testing this thing2"
      }
    ]
  },
  {
    "id": "foo3",
    "name": "Test3",
    "description": "random test3",
    "languages": [
      "en"
    ]
  }
]

Let say I have an array object like this above, If I do a map over it, it is giving me an error because I don't have a subThemes key in the 3rd object.

const children = theme.subThemes
      .map(subTheme => {
        const { subThemes = [] } = aggregations;  
              ...rest of the code}

TypeError: Cannot read property 'map' of undefined

How do I bypass the undefined error? Thanks in advance.