Develop Reference

Vscode available properties intellisense using javascript or typescript in a function which's parameter is a string

Look here.
For the first parameter of the addEventListener function, vscode gives me several inbuilt suggestions
How should I make this available in my javascript function?
It could be using Jsdoc or typescript etc.

You can always see what vscode uses for intellisense by hovering on the addEventListener method or ctrl + click to go to the definition in lib.dom.d.ts file where all definitions are present.
By doing the above you will see that the vscode uses the keys of a class called WindowEventMap.
So your function accepting a event listener name could be
function myFunc(event: keyof WindowEventMap): void {
}
A .d.ts file contains typings for java script code. This is how you get typings like the one in your question and from some packages published to npm.
How is your javascript function used by others?
Are you creating a javascript library which will be used by others? Then either use typescript during development and let typescript generate .d.ts files for your code for you. Or If you are not using typescript for development you will have to create your own declarations. And publish your library with these .d.ts file(s). Refer to DefenitelyTyped project for examples.
A file in the same project which uses this function? If you are using typescript like i mentioned in the code snippet above, then intellisense will automatically be shown for users. Else if you are using javascript, then you can still try and create .d.ts files and refer to them in your ts config (Haven't tried this myself).

Kotlin JavaScript to TypeScript Definition File

I have found the ts2kt library which will create Kotlin header files from arbitrary .d.ts files. But, I want to go in the opposite direction.
I want to build a Kotlin library that will compile to JavaScript, but I want to use it from TypeScript. Is there a way to make Kotlin generate a .d.ts file(s) from its exposed interfaces? Am I approaching this the right way?

In Kotlin 1.4-M1 support for exporting TypeScript definitions was added
Preview: TypeScript definitions
Another feature in the new Kotlin/JS IR compiler we’re excited to show off is the generation of TypeScript definitions from Kotlin
code. These definitions can be used by JavaScript tools and IDEs when
working on hybrid apps to provide autocompletion, support static
analyzers, and make it easier to include Kotlin code in JS and TS
projects.
For top-level declarations marked with #JsExport (see above) in a project configured to use produceExecutable(), a .d.ts file with the
TypeScript definitions will be generated. For the snippet above, they
look like this:
// [...]
namespace blogpost {
class KotlinGreeter {
constructor(who: string)
greet(): string
}
function farewell(who: string): string
}
// [...]
In Kotlin 1.4-M1, these declarations can be found in build/js/packages/<package_name>/kotlin alongside the corresponding,
un-webpacked JavaScript code. Please note that since this is only a
preview, they are not added to the distributions folder by default for
now. You can expect this behavior to change in the future.

I'm looking to do something like this to share models between Android and JS. There is the ts-generator library, which takes jvm classes and generates ts definitions from them.
I haven't tried this yet, but it theoretically should work. Separate the API for your library (or an interface the api implements) into a kotlin common module. You can then compile the api into a jvm module, and run it through the ts-generator to generate ts definitions.
Maybe there is a clever way to do this to skip the compilation to jvm for ts definition generation.

How to write typescript definition files for new library?

If you were to write your own open source typescript library how would you structure the typescript and the typescript definitions? From what I understand the definition files are really there for the compiler so that when your library gets consumed there is intellisense to help the consumer.
However, how do definition files work internally when developing a library? Would you put all of your types (interfaces, classes, etc) in a {module}.d.ts file and reference the definition file internally so that you don't have to write the types twice (once in the {module}.ts and once in the {module}.d.ts?
In the typescript compiler options what is the point of setting "declaration" to true? To me it doesn't seem helpful to automate the definition file creation if it won't be very helpful (i.e. comments).
If your library has multiple modules would you write a separate definitions for each module and add references between definitions then concatenate them with some automater (i.e. gulp, grunt)?
What is the recommended way to expose your library so that both typescript consumers and javascript consumers can use your library?
How would you write tests in typescript? Would you import the typescript modules in your test files?

When developing your library in typescript you do not use definitions of your own classes. They are 'self contained' and already provide all type information to transpiler/IDE. The definitions are as you have mentioned for external consumers.
The point of setting declaration to true is to get rid of tedious job of collecting type information from typescript classes and pasting it in definition files. As I have said this information is already present in typescript files and can be automatically extracted by transpiler.
Regarding the way of how to organize your definitions and expose them - I am sure there are multiple different ways, one thing for sure - you do not want to write the same class definitions manually in two places. Having said that I can give you a sample of how I expose library project and consume it in another one.
Pay attention to index.ts files there and how the root one is exposed in package.json -> typings field. There you will also find exampls of unit tests for the library via jasmine.
Hope this will give you some information of how to go process with your library development.

Reference javascript file inside typescript

I would like to use a javascript function inside a typescript file. How can I reference the javascript file containing that function inside typescript file?
I tried
<reference path="../libs/myjsfile.js" />
this brings an error while building like: Cannot resolve referenced file: ../libs/myjsfile.js

While the two answers above will work for a small or single variable API, the correct answer is to write a type declaration file for your JavaScript file.
The type declaration file for your example would be named myjsfile.d.ts, where the .d.ts suffix is what tells the TypeScript compiler that it's parsing a declaration file. If you use Visual Studio, your declaration file will be recognized by the TypeScript compiler as long as it is somewhere (anywhere) in the project you are working on.
Declaration files store metadata about JavaScript variables, functions and objects so the TypeScript compiler can do its job. If your JavaScript file happens to be a library or framework in common use, DefinitelyTyped is definitely the place to find the definition file you need.
If your JavaScript is your own work or less well known, you'll have to write your own type declaration file. See the section Writing .d.ts files in The TypeScript Handbook.
If the name declaration file sounds intimidating, don't worry - they are far easier to write than programs. Also, remember that you don't need to specify all metadata for your JavaScript file, just the functionality you need.
Let me know if you need any help!

You just have to tell the compiler that the method exists outside typescript:
declare function myJavaScriptFunction(param1: number, param2: JQuery): void;
Be sure that the website loads all files needed, the normal js files and the js files that the typescript compiler generated

You can see the latest version of the TypeScript documenttion in TypeScript
Handbook on GitHub:
Working with Other JavaScript Libraries
Writing Definition Files
Also I am recomending you the tsd utility to search and install TypeScript definition files directly from the community driven DefinitelyTyped repository. This repository contains definition files *.d.ts for the most popular javascript libraries.

this brings an error while building like: Cannot resolve referenced file: ../libs/myjsfile.js
Reference file tags are for TypeScript not JavaScript. You would load JavaScript using a script tag in your page (or something like requirejs).
To use the JavaScript from TypeScript you would declare the API of the javascript you want to use for TypeScript consumption (as AKR pointed out).

What is this JavaScript reference syntax used in Visual Studio?

In Visual Studio 2012, I created a web application then found the following line in the default _references.js script that came with the project:
/// <reference path="jquery-1.8.2.js" />
What is this reference notation doing? This is confusing - isn't this just a comment, which shouldn't do anything? As I understand, a double slash (//) comments out a line in JavaScript. Is there anything special about triple-slash comments?

see this article
http://msdn.microsoft.com/en-us/library/vstudio/bb385682.aspx
and find Reference Directives
A reference directive enables Visual Studio to establish a relationship between the script you are currently editing and other scripts. The reference directive lets you include a script file in the scripting context of the current script file. This enables IntelliSense to reference externally defined functions, types, and fields as you code.

It's a Triple Slash Directive and has many uses.
See this article
https://www.typescriptlang.org/docs/handbook/triple-slash-directives.html
... Triple-slash references instruct the compiler to include additional files in the compilation process....
Very useful in linking typings definitions... for example, the mysql.d.ts typings has the following reference to the node.js typings...
///<reference path='../node/node.d.ts' />