What is Aurelia's .two-way command for?

One of my book’s reader sent me an email earlier this week. He was confused by a statement from the book and thought it might contain a typo or some kind of error. This statement said that the .two-way binding command didn’t adapt to its context, while .bind did.

I answered that it was no error, that .bind uses two-way binding when the target property supports it and falls back to one-way binding when it doesn’t, while .two-way forces two-way binding. Although it may not be as clear as I intended, that’s what I meant when I wrote that it didn’t adapt to its context.

That question made me realize that at first glance, .two-way seems pretty useless. Why would someone want to force two-way binding on a property that doesn’t support it? It makes no sense.

I gave it some thought, and found a use case where it does actually make sense.

The .bind command uses the target property’s default binding mode. Aurelia considers that the default binding mode for all native HTML elements’ attributes and properties is one-way, except for properties affected by user inputs on form-related elements, such as an input’s or a select’s value, whose default binding mode is two-way.

However, when creating custom elements and attributes, you have complete control over the default binding mode of your component’s properties. One could easily imagine a custom attribute - or a custom element’s property - whose default binding mode is one-way, but which supports two-way binding by being somehow related to user input. In such a case, using the .two-way command would be the only way to force two-way binding for this property, since the .bind command would use one-way binding by default.

Such a scenario doesn’t seem very likely to me, but Aurelia nevertheless supports it.

Thanks to Christian for his question.