What is the significance of `(<typeof className>this.constructor)` in TypeScript?

After inspecting the source code of jQTree, written in Typescript, available at https://github.com/mbraak/jqTree, I came across the following snippet:

export default class SimpleWidget{
            protected static defaults = {};
            public options:any
            protected $el : JQuery

            constructor(el:Element,options:any){
                this.$el = $(el);
                console.log(typeof SimpleWidget)
                let defaults = (<typeof SimpleWidget>this.constructor).defaults;
                console.log(defaults)
                this.options = $.extend({},defaults,options);
            }
        }

Can you explain what (this.constructor) means? How does it access the static property after assertion?

javascripttypescript

Answer №1

In this scenario, the class being discussed will ultimately become a constructor function, with the static property directly linked to it:

function CustomWidget(element, options) { ... }

CustomWidget.defaults = {};

If there are child classes involved, they would specify their own set of defaults:

class SpecializedWidget extends CustomWidget {
  protected static defaults = { setting: 'specialized default' };
}

This setup translates to:

SpecializedWidget.defaults = { setting: 'specialized default' };

Hence, in order for the parent class to access the child's specific static defaults, it should utilize this.constructor.defaults instead of just relying on CustomWidget.defaults, as the former approach is more dynamic.

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