The migration to TS will probably occur on two different branches :

• what we can do on the 5.x branch should be done here, without altering the dist output. It has been started in allow TS files to be compiled in src dir #2054
• what can not be done here should be done on the 6.x branch.

A list of things already spotted that won't be possible on v5 :

• buble is our actual ES5 compiler, and does not works well with typescript output or even modern code (string interpolation, class props, etc.), : Replace buble by babel #2050
• In the immutable codebase, the class constructor are factories that can create instances of other classes. This is not possible in modern JS. (See Avoid return in constructors #2041 )

The purpose of that is to make the full migration the less risky possible by doing everything possible in the v5 branch.