how called language that used in .ts? look like javascript with weirdness, and where it documented?
You mean angular 2? typescript? font style?
is it correct:
ionic based on angular and cordova, used typescript
can i write ionic code just in plain javascript?
because writing javascript is a pain the a** 
ts files = typescript is what your looking for
typescript is a trancoding language, it allow to write structured code which gonna be, at build time, converted into javascript code.
typescript => ionic cordova build or ionic serve => javascript
for example, I amost didn’t know anything about javascript before begining my project, but I know java. Using typescript allowed me to easily start to learn Ionic because writing typescript is almost like writing java, you could use class, abstraction, variables, methods, etc.
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there you could find a more graphical way of understanding it:
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Once you understand what TypeScript (Yes that language is TypeScript) is solving, you’ll surely start to love that weirdness 
Basically, as @reedrichards pointed out, TypeScript lets us write OOP code with much more ease than the prototype based syntax javascript uses.