Develop Reference

TypeScript/JavaScript in Angular

I'm a beginner level in writing in TypeScript and working with Angular, but I am experienced in JavaScript. From what I've read, any TypeScript would be translated into JavaScript and compiled. So when I code in the TypeScript file (e.g. the whatever.component.ts file), does that mean I can mix or introduce JavaScript language code inside where I also write TypeScript in the .ts file?
How does this all exist within the .ts file in regards to a code readability aspect for someone who is just learning TypeScript/Angular?

You can of course use JavaScript anyhow you like it. However, TypeScript is built to help the developers maintaining projects by using variables types. The types can then easily be read and previewed by good IDE with the appropriate extensions.
Get used to adding types. Use interfaces for custom objects, embrace the Angular way and you will forget you ever had to deal with JavaScript very soon.
A good base for productive IDE would be Visual Studio Code with the Angular Essentials extension.
Welcome to the Angular community!

Package dart lib into single javascript file to make lib available in websites

I wrote my first library in dart. Now I want to make use of it in a website. My idea was to compile all necessary dart code of my lib and its dependencies into one javascript file that has a useful "global" API. As I understand this I would also write this API in dart and compile it altogether to javascript but I fail to see how this is done. The https://www.dartlang.org/tools/dart2js/ wasn't particular helpful to me.
To give a simplified example: The library is a generic parser controlled by a grammar. A parse-tree is build from some input file and a grammar. Think of it like:
ParseTree parse(File input, File grammar);
So in the resulting javascript I want to have this available in some form so one could write e.g.
var tree = MyParserLib.parse("path/to/input.file", "my.gramamar.file");

Usually you build your entire app at once. Building distinct parts or libraries to JS and using the output in another app is not (yet?) well supported.

How to port Javascript project to Coffeescript

If I wanted to port an existing project from Javascript to Coffeescript (in my case, within a Rails application), would I need to convert existing Javascript files? I'm worried about converting really large and CDN hosted files like jQuery and jQueryUI. How would I work around that?

As #asawyer stated, you do not need to port existing JavaScript libraries and such to CoffeeScript.
CoffeeScript exists to add convenience for writing your own custom code. Because the CoffeeScript compiles into JavaScript, it plays nicely with other JavaScript libraries like jQuery without your needing to convert those libraries into CoffeeScript.
If you have your own code that you want to convert into CoffeeScript, I've found js2coffee.org very helpful. It also serves as a great learning tool for "thinking in JavaScript" and seeing how it would be done in CoffeeScript.

Within rails, coffeescript is run through a compiler and into javascript anyway. You can include pure javascript files if you want (.js), or coffeescript (.js.coffee), which is compiled into js. If you're just looking to include legacy files, you won't need to do any conversion at all -- Might not make sense to convert to coffeescript, just to have rails compiler convert back to javascript in the asset pipeline... See http://guides.rubyonrails.org/asset_pipeline.html#coding-links-to-assets for more from the rails team on how to use js in the asset pipeline.
jQuery is already included in newer versions of Rails (3.1 onward), so that shouldn't be an issue. http://weblog.rubyonrails.org/2011/4/21/jquery-new-default/
Also, many other jQuery addons have a gem that you can simply include -- no need to reinvent the wheel there.

What javascript/php template libraries will parse the same template files?

I originally was using Mustache.js but found that it does not work well for rendering large nested trees (it has no way of disambiguating variables of the same name in nested structures).
I was happy to find a PHP version of Underscore.js, but when I looked at the code of Underscore.php I realized that its template method does not render Underscore.js-style templates. Instead it replicates similar functionality but with PHP variables.
Jquery-tmpl/jquery-tmpl-php is another template language with JS and PHP libraries, but my concerns are that the jquery-tmpl-php library seems not to be used much (very few people are following it on github) and that jQuery decided to remove jQuery-tmpl as an official plugin: http://blog.jquery.com/2011/04/16/official-plugins-a-change-in-the-roadmap/
Also it seems that the author of jquery-tmpl has not touched it in months.
What are other people doing to render Javascript and PHP using the same templates?

Jade does it:
https://github.com/everzet/jade.php
https://github.com/visionmedia/jade

You should try mustache. It's has implementations in many different languages.
Have to get used to a different way to do control structures but its not too difficult to figure out.
http://mustache.github.io/

Javascript Templating with Django

I'd really like to use a Javascript templating system together with Django. The syntax and style of Mustache.js (and it's derivatives) really sits well with me. The problem is the delimiter tag used by Mustache doesn't play nicely with the syntax of Django's templating system.
Is there any good way to use them together?
I have tried using this verbatim snippet to render the JS templates properly. The problem with that solution is I still sometimes need Django variables or URLs inside the JS.
I have also tried changing the delimiter for Mustache using
{{=[[ ]]=}}
However, that doesn't allow for using section tags, like {{#}}. The author has said he intends on removing that capability in future releases altogether.
Are there any template libraries for Javascript that follow closely to Mustache.js, but use different delimiters? Or is there another solution for changing the delimiters Mustache.js uses?

I've used jquery's templating with django. Ultimately I decided the best way is:
put all the javascript into static javascript files and serve them up without any serverside processing
in the django templates deliver all the html hooks (id's or classes) but no js.
in the js use 'document.ready' plus jquery selectors to insert tags into the page and attach events
if the js needs data then make an ajax call.
I softened on the last one and might embed data as a block of json into the django template, and perhaps also have a short js at the bottom of a template which do no more than set variables/parameters to instruct the js how to render the page - little clues from the server side telling it what to do.
Thus:
contention between the escape characters becomes a non-issue
you don't have to keep asking yourself "is this code running on the server or on the client" because you're not trying to write both at once into the one file
your javascript code is necessarily better structured and re-usable

It looks like it would be extremely easy to change Mustache's delimiters, although configurable delimiters should be supported in my opinion.
If this is not acceptable to you, there are many other templating libraries out there such as jquery-tmpl and underscore.template.

EJS is a pretty nice templating system. It uses <% %> tags.
Edit: More templating libs