Is there a reverse logical nullish assignment?

Question

So the nullish coalescing assignment operator ??= assigns the value to the variable only if the current stored value is nullish.

Maybe I'm missing the obvious but I can't think of a slick solution (without if statements) to only assign if the right-hand side value being assigned is not nullish?

I'm using nodeJS to give a bit more context.

---

I want

let x r??= 2;
// Updates 'x' to hold this new value
x r??= undefined;
// Has no effect, since the value to assign is nullish
console.log(x); // 2

EDIT to give more clarity to my problem:

I want a variable only to be assigned a new value, if that new value is not nullish.

let iceCream = {
    flavor: 'chocolate'
}

const foo = 2.5
const bar = undefined;

iceCream.price r??= bar
// does not assign the new value because it is nullish
console.log(iceCream.price) // expected to be error, no such property

iceCream.price r??= foo
// assigns the new value because it is not nullish but a float
console.log(iceCream.price) // expected to be 2.5

iceCream.price r??= bar
// does not assign the new value because it is nullish
console.log(iceCream.price) // expected to still be 2.5

Lelandleler

Doesn't x ??= undefined already do exactly that? – Zondra

@Zondra x ?? = undefined assigns x to undefined if x is nullish. OP wants to assign if x is not nullish. – Scar

@DanielBaldi: I think it’s if the new value is not nullish. – Postmark

@Proxycon: Show the context where you’re going to use it for clarity and most appropriate alternatives? – Postmark

@Postmark which new value you're referring to? – Scar

As I understand it (and as I think Ry is referring to), OP wants to set x to the right hand side operant, if the right hand side operant is not nullish (not when x is not nullish). Though my initial comment wont work, because it will still remain unmodified if both values are not nullish (if x is 2 and you use x ??= 3, it'll remain 2 instead of assigning 3). – Zondra

Regarding "expected to be error, no such property", note that accessing non-existing properties of an object doesn't throw errors. It just returns undefined. (Accessing properties of the undefined value/property however does throw errors.) – Zondra

I believe you're looking for x = newVal ?? x;. Assigns newVal if newVal is anything except null or undefined, even falsy values (0, NaN, etc...). In all other cases, assigns x, which keeps it unchanged. For objects you can do x.foo = newVal ?? x.foo;, obviously. – Xever

Answer 1

No, that’s not a single operator. The closest is two operators:

x = undefined ?? x;

Postmark

This comes close but does not work if I want to set a new property on an object. – Lelandleler

@Proxycon: You need an if or a function wrapping one, then. – Postmark

Answer 2

Adding another answer after clarification as editing my previous one seemed weird.

The simplest way I can think of a solution without if is as follows:

let iceCream = {
    flavor: 'chocolate'
}

const foo = 2.5
const bar = undefined;
bar && (iceCream.price = bar)
// Another possible solution if creating the property with a nullish value is ok for you:
iceCream.price = bar || iceCream.price;

Scar

This is better than your other answer, but non-nullish is not the same as 'truthy', and nullish is not the same as 'falsy'. This distinction is very important to the question, since OP wants right-hand values such as 0, "" and false (which are evaluated as falsy) to be assigned. – Isbella

Answer 3

You could use logical AND assignment.

From the MDN Web Docs:

let a = 1;
let b = 0;

a &&= 2;
console.log(a);
// expected output: 2

b &&= 2;
console.log(b);
// expected output: 0

Scar

But this operator also only depends on the value stored in the variable, not on the value that is to be assigned if I understand correctly. – Lelandleler

This answer doesn't really address the issue. This only takes into consideration the left-hand side of the assignment. As explained in other comments, OP wants the assignment to take place if the right-hand side (i.e. the new value) is non-nullish. – Isbella