Trying to use a variable in a json file path – javascript

I’m trying to use a variable to access a field in my json file,
how do I do something like this?
js:

let file = require('./file.json')
let someVar = 'firstField'
console.log(file.firstObject.{someVar})

json:

{
  "firstObject": {
     "firstField":"firstValue",
     "secondField":"secondValue",
     "thirdField":"thirdValue",
  },
  "otherObject": {
     "otherField":"otherValue"
  }
}

desired outcome :

'firstValue'

>Solution :

obect[str] is what you looking for.
Your code should look like this:

let file = require('./file.json')
let someVar = 'firstField'
console.log(file.firstObject[someVar])