Why doesn’t JavaScript have a shorthand for adding values with null treated as zero?

That's an interesting approach, but there are likely existing proposals that could address this without altering the language dramatically, such as the upsert proposal. It's worth checking out!

I see your point, but what if I want a different default value rather than zero for a missing key? A universal solution could be adding default value support in Maps, like a get method that allows specifying a default. This might improve both convenience and performance!

A null value often indicates a mistake in code rather than an acceptable operand. Developers have to be cautious with such cases, so keeping the default behavior as it is may help prevent bugs. Plus, the idea of making the += operator automatically treat null as zero could confuse less experienced developers and lead to unexpected results.

You can actually simplify this a bit with the nullish coalescing assignment operator (??=). Try this:

obj[item.id] ??= 0;
obj[item.id] += item.amount;

It doesn’t change the behavior of += but offers a clearer syntax.

Changing the behavior of the += operator to treat undefined or null as zero would be a backward-incompatible change. Right now, undefined + 5 gives NaN, and that’s a behavior that many existing codebases depend on. Using existing tools like ?? or ??= keeps things safer and more predictable.