Nativescript data-binding function calls not working

In reference to this link https://docs.nativescript.org/core-concepts/data-binding#supported-expressions

function calls  myFunc(var1, var2, ..., varN)   Where myFunc is a function available in binding context (used as context for expression) or within application level resources. The value of the var1 and varN will be used as parameter(s).

Im using a RadList where for each item i have a Label where i need to show a composed string checking complex logics based on item params.

Can anyone give typescript example how we can use function calls. I tried many ways but nothing worked.

As Culita suggested - using $parent should work, but it doesn't. This seems like a bug to me, make sure to open a new issue about it in the respective GitHub repository.

As a workaround, you can create the function in the type you are using to bind the template to. In this case - it works as expected:

class Location {
    constructor(city: string, country: string, imageSrc: string) {
        this.city = city;
        this.country = country;
        this.imageSrc = imageSrc;
    }

    city: string;
    country: string;
    imageSrc: string;

    getLabel(args): string {
        return "a" !== args ? "My label" + args : "Your Label ? " + args;
    }
}

Take a look at the modified Playground.