TODO

* Next
 
[ ] add JS primitives
  [x] cljs.core.typed/JSUndefined
    * 'undefined' in TypeScript
    * idea 1: nil expands to (U nil Undefined) internally, but always elide Undefined
      when pretty-printing a union that includes both nil and Undefined.
      * This to avoids: 
        Expected: nil, Given: nil
        since 
        Expected: (U Undefined nil), Given: nil
      * Problem: lots of special casing for Nil? in the type checker
      * support nil?/undefined?/some? predicates
      * recognize undefined as a false value
      * (= x nil) ;=> (is x (U nil Undefined))
    * idea 2: New type JSNull
      (U JSNull JSUndefined) <: nil
      nil <!: JSNull
      nil <!: JSUndefined
      * JSNull reserved mostly for interop
      * JSNull pprints to nil
        (inc nil)
        ; Error:
        ; Expected: Number
        ; Given: nil
      * nil :- JSNull
        (ann f [JSNull -> Any])
        (f nil)
      * structural equality on `nil` should be enhanced
        (let [x :- JSNull nil
              y :- nil nil
              z :- JSUndefined ...]
          (= x y z))
        ; true ; tt | ff
    [x] support undefined? predicates
      * unit tests for 'if'
  [*] cljs.core.typed/JSNumber
    * 'number' in TypeScript
    * (ann number? (Pred JSNumber))
  [*] cljs.core.typed/JSBoolean
    * 'boolean' in TypeScript
  [*] cljs.core.typed/JSString
    * 'string' in TypeScript
  [*] cljs.core.typed/JSSymbol
    * 'symbol' in TypeScript
[*] add cljs.core.typed/JSObject
  * any non-primitive type
  * 'object' in TypeScript
    * https://blog.mariusschulz.com/2017/02/24/typescript-2-2-the-object-type
  * 'Object' in Google Closure (or 'Object<?,?>' maybe?)
    * https://github.com/google/closure-compiler/wiki/A-word-about-the-type-Object
[*] rename int => cljs.core.typed/JSInteger
  * for `integer?`
[ ] create JSObj type
  * analogous to HMap
  * Use undefined to signal optional
  * (JSObj {:a (t/U Node t/JSUndefined)
            :b})
[ ] implement ann-js
[ ] implement ann-jsclass
[ ] implement ann-jsinterface
[ ] implement ann-jsalias
[ ] support js/undefined top-level builtin classes
  * eg. js/String, js/Symbol, js/Function
  * Note. js/Object is 'Object' in TypeScript
  * Note. js/Function is 'Function' in Google Closure
  * Note. js/Array is 'Array' in Google Closure
  * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects
  * https://funcool.github.io/clojurescript-unraveled/
[ ] add common Regex annotation for #""
[ ] remove cljs.core.typed/IPersistentMap etc.
[ ] remove ArrayCLJS
  * replace with js/Array
[ ] remove FunctionCLJS
  * replace with js/Function
[ ] fix cct.subtype/resolve-JS-reference function
[ ] remove letfn> etc.
  * replace with non-> versions
* rename primitives to lower case (or namespaced in `js`?)
  * JSboolean, JSsymbol, JSstring, JSnull, JSundefined
  * js/boolean, js/symbol, js/string, js/null, js/undefined
* handle special Closure types
  * possibly prefix with `js`?
  * js/IObject, js/IArrayLike, js/IThenable

# Later

* add heterogeneous object type
  * optional entries
* delete Array stuff for CLJ/CLJS
* revise default js-nominal mappings
* pprint JSUndefined and JSNull to `nil` when appropriate
* `boolean?` tests for primitive boolean