add FAQ for language

docs/javascript.md

@@ -27,5 +27,10 @@ Visit the cards below to starting programming JavaScript and TypeScript with the
 },{
   "name": "Classes",
   "url": "/js/classes"
-}]
+},{
+  "name": "FAQ",
+  "url": "/js/faq"
+}
+]
+
 ```

docs/js/faq.md

@@ -0,0 +1,58 @@
+# Frequently asked questions
+
+# What is the language supported for the micro:bit?
+
+For the micro:bit, we support a "static" subset of TypeScript (itself a superset of JavaScript):
+
+## Supported language features
+
+* variables with `let`, `const`, and `var`
+* functions with lexical scoping and recursion
+* top-level code in the file; hello world really is `console.log("Hello world")`
+* `if ... else if ... else` statements
+* `while` and `do ... while` loops
+* `for(;;)` loops (see below about `for ... in/of`)
+* `break/continue`; also with labeled loops
+* `switch` statement (on numbers only)
+* `debugger` statement for breakpoints
+* conditional operator `? :`; lazy boolean operators
+* namespaces (a form of modules) 
+* all arithmetic operators (including bitwise operators); note that in microcontroller targets
+  all arithmetic is performed on integers, also when simulating in the browser
+* strings (with a few common methods)
+* [string templates](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals) (`` `x is ${x}` ``)
+* arrow functions `() => ...`
+* classes with fields, methods and constructors; `new` keyword
+* array literals `[1, 2, 3]`
+* enums
+
+## Unsupported language features
+
+We generally stay away from the more dynamic parts of JavaScript. 
+Things you may miss and we may implement:
+
+* exceptions (`throw`, `try ... catch`, `try ... finally`)
+* `for ... of` statements
+* object literals `{ foo: 1, bar: "two" }`
+* method-like properties (get/set accessors)
+* class inheritance
+
+For JS-only targets we may implement the following:
+
+* regular expressions
+* classes implementing interfaces
+
+Things that we are not very likely to implement:
+
+* file-based modules (`import * from ...`, `module.exports` etc); we do support namespaces
+* spread operator
+* `yield` expression and ``function*``
+* `await` expression and `async function`
+* `typeof` expression
+* tagged templates ``tag `text ${expression} more text` ``; regular templates are supported
+* binding with arrays or objects: `let [a, b] = ...; let { x, y } = ...`
+* `with` statement
+* `eval`
+* `delete` statement
+* `for ... in` statements
+* JSX (HTML as part of JavaScript)