Add 2.x option to treat complex observers as they were in Polymer1

[beckysiegel]

In order to smooth the upgrade process, add an option at the element level to not call observer/computed functions if any of the arguments is undefined.

Example:

properties: { foo: String, bar: String, baz: { type: String, computed: computedFn(foo, bar), }, usePolymer1StyleObserver: true, }, computedFn(foo, bar) { return foo + bar; }

[jsilvermist]

Why not just add at the beginning:

if (!foo || !bar) return;

Or

if (foo === undefined || bar === undefined) return;

[web-padawan]

You can also do it like this:

if (Array.from(arguments).some(arg => arg === undefined))

[TimvdLippe]

I am inclined to agree with @jsilvermist and @web-padawan here. It should be quite straightforward to handle these cases. This also makes the logic more explicit. I would therefore be against adding such an option.

[beckysiegel]

Sorry, I didn't illustrate the real problem very well. It's very clear in this case that you can add:
if (foo === undefined || bar === undefined) return;

However, I've worked on upgrading an app with many elements, and thousands of places where this can happen, many not as obvious as this. A few examples:

• An property that has a default value set, but is also two way bound to its parent. It can be undefined when not expected
• Computed values in templates

This issue is a huge pain point for large apps with many async properties. My goal is to make it easier to incrementally add the undefined checks as you are suggesting.