Use optional chaning to increment object property

Question

I want to increment an object value using the optional chaning operator (?.). In my example I want to increase the age of a Person by one in a function called birthday() only if an age is set for the current person. The line with the if clause works fine (also tenary if works obviously). However I wanted to know if something like the line below is possible. The syntax given results in an SyntaxError: invalid increment/decrement operand. Maybe its an issue based on the operator precedence, but I have now clue how to solve it.

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}
let peter = new Person("Peter", 27);
let paul = new Person("Paul", undefined);

Person.prototype.birthday = function() {
  if (this.age) this.age++;
};

//Person.prototype.birthday = function(){this?.age++};

peter.birthday();
paul.birthday();
console.log(peter, paul);

T.J. Crowder · Accepted Answer · 2022-11-24T16:44:00.963

I want to increment an object value using the optional chaning operator (?.)

You can't. The reason is that the result of optional chaining is just a value, not a property reference, so you can't use any of the operators that write back to the property (like ++, --, +=, etc.). (That's shown by this part of the specification.) In contrast, the result of a property acccessor like this.age is (in specification terms) a reference to the property, so the operator can write back to the property.

Your version with if is the way to handle this.

But separately, just as a side note, your two statements do different things.

Your optional chaining (this?.age) is checking that this is not nullish. But your if is checking that this.age is not nullish. The equivalent if would be if (this) { this.age++; }. But from the overall code context, you don't want that, you want what you currently have in the if, though I think I'd probably make it a more specific check:

if (this.age !== undefined) {
    this.age++;
}

I say that because 0 is falsy, so just if (this.age) would not increment 0.

Use optional chaning to increment object property

1 Answers1