Implicitly get bindable default value from assignment

[Justin-Lessard]

I would be nice that initial variable assignment would be automatically set as the default value for typed bindable/observable.

What i mean is the following syntax:

@bindable.number public myNumber: number = 123;

Should be equivalent to this one:

@bindable.number({defaultValue: 123})
public myNumber!: number;

Now the first version without the default value create the bindable with an undefined value at first, and then assign 123 to it.

This cause problems since have a strict validation in my coercion function that throws an error if I try to assign undefined.

How hard would it be to implicitly set a default value for defaultValue using the part after the =?

Thank you.

[bigopon]

I missed this, so sorry for the delayed answer.

The issue you described is because the way TypeScript compiles to JS, which I can't fix.

strict validation in my coercion function

May I know how you are using it ?

[Justin-Lessard]

Thanks for your answer!

By strict validation function, I means I throw an error when the value is not a valid type. See a simplified example below.

coerceFunctions.number = (value: any): number => {
    const numeralValue: number = Number(value);

    if (isNaN(numeralValue)) {
        throw new Error(`"${value}" is not a valid number`);
    }

    return numeralValue;
};

Anyway this is just a minor annoyance, otherwise your plugin works flawlessly 👍
So, feel free to close this issue. Thank you for the great work!

[bigopon]

@Justin-Lessard What you described is also my concern, as I'm not sure whether initial value should be coerce no matter what, even when not explicitly defined:

class A {
  @bindable.number()
  a: number;
}

Should it coerce undefined (starting value) or not ? At the moment, it is coerced into NaN

[Justin-Lessard]

We use your plugin to coerce value from binding to the view-model into the correct type. I don't see a case where coercing the initial value make senses.

In all case you could just explicitly declare your variable in the correct type instead of using a coerce function.

In your example, if you really needed to have NaN and not undefined you could just declare a = NaN.

But Imho, it's cleaner to explicitly assign NaN than using an initial coercing. Moreover, in your example where a is undefined, you would need to change the type to number | undefined or skip initial validation using a!: number to pass the typechecker.

Worst case, would it be hard to add a config to toggle the initial coercion on or off?

[bigopon]

Turning off them by default is not hard, maybe a check from exposed config here https://github.com/aurelia-contrib/aurelia-typed-observable-plugin/blob/master/src/patches.ts#L117, and here https://github.com/aurelia-contrib/aurelia-typed-observable-plugin/blob/master/src/patches.ts#L158 to disable them.

Disabling it by default sounds like a breaking change to me, probably a config is needed to turn off instead.