How to print sub-items of a key?

I have a file, assets.json:
```
{
 "window":{
         "bottom":"_"
 }
}
```

I tried to cat the file while piping it with JSON:
`cat assets.json | json window bottom`
which outputs:
```
{
  "bottom": "_"
}
```
I want it to just output the value of the  key `bottom`.

Like:
`_`