Self-referenced JSON?

TL;DR if you are making a foo.json file then I can't help you! Otherwise, read on.

Wouldn't it be handy if we can write something like this?

const person = {
    firstName: 'George',
    lastName: 'Costanza',
    fullName: `${person.firstName} ${person.lastName}`
};

console.log(JSON.stringify(person));

error TS2448: Block-scoped variable 'person' used before its declaration.

It doesn't work - because when person.firstName is evaluated, person is not available yet.

Workarounds include declaring firstName and lastName separately before person, which is not as nice.

const firstName = 'George';
const lastName = 'Costanza';

let person = {
    firstName,
    lastName,
    fullName: `${person.firstName} ${person.lastName}`
};

or make person a variable (vs a const value) and initialise fullName as a second step. e.g.

let person = {
    firstName: 'George',
    lastName: 'Costanza',
};

person.fullName = `${person.firstName} ${person.lastName}`;

Not the end of the world but it annoys me a lot as I really want person to be immutable.

There is also the option of making a class Person, a bit too heavy-handed for my taste.

Not all hope is lost. The trick is to make fullName a getter.

const person = {
    firstName: 'George',
    lastName: 'Costanza',
    get fullName() { return `${person.firstName} ${person.lastName}`; }
};

console.log(JSON.stringify(person));

{"firstName":"George","lastName":"Costanza","fullName":"George Costanza"}

It works because by the time the fullName getter is evaluated, person is now available. The benefit of lazy evaluation.

Be warned though, if you are already likening it to something like this.firstName + this.lastName.

const suzanne = { ...person, firstName: 'Suzanne' };
console.log(JSON.stringify(suzanne));

{"firstName":"Suzanne","lastName":"Costanza","fullName":"George Costanza"}

It makes sense - the getter still uses person.firstName, not suzanne.firstName. So suzanne will need its own fullName getter. No surprise, and definitely not enough reason to convert to class.