A quick note on types in Deno+TypeScript

Understanding the plumbing of a program that is built with Deno in TypeScript can be challenging if you can't identify the type of variables or constants. TypeScript inherits the JavaScript function, typeof. This works nicely for simple types like string, boolean, number but is less useful when compared to a class or interface name of a data structure.

There are three approaches I've found helpful in my exploration of type metadata when working with Deno+TypeScript. (NOTE: in the following the value VARIABLE_OR_CONSTANT would be replaced with the object you are querying for type metadata)

typeof

This is good for simple types but when a type is an object you get [object object] response.

Object.protototype.toString.call(VARIABLE_OR_CONSTANT)

This is what is behind the typeof function but can be nice to know. It returns the string representation of the VARIABLE_OR_CONSTANT you pass to it.

VARIABLE_OR_CONSTANT.constructor.name

This will give you the name derived from the object's constructor, effectively the class name. It doesn't tell you if the the VARIABLE_OR_CONSTANT is an interface. If you construct an object as an object literal then the name returned will be Object.

Here's the three types in action.

  let fp = await Deno.open('README.md');
        console.log(typeof(fp));
        console.log(Object.prototype.toString.call(fp);
        console.log(fp.constructor.name);
        await fp.close()
        
        let t = { "one": 1 };
        console.log(typeof(t));
        console.log(Object.prototype.toString.call(t);
        console.log(t.constructor.name);