Using one object value in another value on the same object Javascript [duplicate]

My question is somewhat simple, but IDK if I haven't made the search with the right keywords, but my problem is:

When defining an object in JS/TS, how can I use one value from the obj into another key, something like:

const person = {
  firstName: 'Régis',
  middleName: 'Faria',
  fullName: `${firstName + middleName}` // Here is where IDK how to do it. I've tried with 'this.' but no success
}

Hope someone can help me out. Thanks in advance!!

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