MakeCode Language: Static TypeScript

PXT programs are written in a subset of TypeScript called Static TypeScript. Currently, we are using TypeScript version 2.6.1. TypeScript itself is a superset of JavaScript, and many PXT programs, especially at the beginner’s level, are also just plain JavaScript.

PXT is meant for teaching programming first, and JavaScript second. For this reason, we have stayed away from concepts that are specific to JavaScript (for example, prototype inheritance), and instead focused on ones common to most modern programming languages (for example, loops, lexically scoped variables, functions, lambdas, classes).

Supported language features

  • variable declarations with let, const
  • 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
  • for ... of statements (see below about for ... in)
  • break/continue; also with labeled loops
  • switch statement (on numbers, strings, and arbitrary types - the last one isn’t very useful)
  • 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 (`x is ${x}` )
  • arrow functions () => ...
  • passing functions (with up to 3 arguments) as values
  • classes with static and instance fields, methods and constructors; new keyword
  • array literals [1, 2, 3]
  • enumerations (enum)
  • asynchronous functions that look synchronous to the user
  • method-like properties (get/set accessors)
  • basic generic classes, methods, and functions
  • class inheritance
  • classes implementing interfaces (explicitly and implicitly)
  • object literals { foo: 1, bar: "two" }

Unsupported language features

Static TypeScript has a more restrictive type checker than TypeScript. In particular, it does not support:

  • explicit or implicit use of the any type
  • union or intersection types
  • interface with same name as a class
  • casts of a non-class type to a class
  • downcasts of a superclass to a subclass
  • extending a class by an interface
  • function parameter bi-variance (parameter subtyping is contra-variant)
  • inheriting from a built-in type
  • using a built-in or generic function as a value
  • this used outside of a method
  • function overloading

Static TypeScript enforces nominal typing of classes, rather than the structural typing of TypeScript.

We generally stay away from the more dynamic parts of JavaScript. Things you may miss and we may implement:

  • object destructuring with initializers
  • shorthand properties
  • exceptions (throw, try ... catch, try ... finally)
  • using generic functions as values and nested generic functions
  • class inheritance for generic classes and methods
  • initializers for class fields
  • public/private annotations on constructor arguments (syntactic sugar to make them into fields)
  • binding with arrays or objects: let [a, b] = ...; let { x, y } = ...
  • delete statement (on object literals)
  • spread and reset operators (statically typed)
  • support of enums as run-time arrays
  • lambda functions with more than three arguments
  • new on non-class types
  • typeof expression

Things that we are not very likely to implement due to the scope of the project or other constraints (note that if you don’t know what a given feature is, you’re unlikely to miss it):

  • file-based modules (import * from ..., module.exports etc); we do support namespaces
  • yield expression and function*
  • await expression and async function
  • typeof expression
  • tagged templates tag `text ${expression} more text` ; regular templates are supported
  • with statement
  • eval
  • for ... in statements (for ... of is supported)
  • prototype-based inheritance; this pointer outside classes
  • arguments keyword; .apply method
  • JSX (HTML fragments as part of JavaScript)

For JS-only targets we may implement the following:

  • regular expressions

Note, that you can use all of these while implementing your runtime environment (simulator), they just cannot be used in user’s programs.

Semantic differences against JavaScript

As such, it isn’t really feasible to run a full JavaScript virtual machine in 3k of RAM, and thus PXT programs are statically compiled to native code to run efficiently. There are two compilation strategies available - the legacy strategy used by the current micro:bit target, and a tagged strategy used by the upcoming SAMD21 targets, as well as all the other targets going forward (possibly including new version of the micro:bit target).

In the legacy strategy, there are some semantic differences with JavaScript, particularly:

  • numbers are 32 bit signed integers with wrap-around semantics; in JavaScript they are 64 bit floating points
  • JavaScript doesn’t have types, and therefore every value can be undefined or null (which are two different values, distinct from 0 or false); in PXT 0, false, null, and undefined all have the same underlying representation (32 zero bits) and thus will test as equal

Execution environments

PXT programs are executed in at least three different environments:

  • microcontrollers, with native code compilation (ARM)
  • browsers
  • server-side JavaScript engines (node.js, etc)

We refer to the browser execution environment as the “simulator” (of the microcontroller), even though for some targets it’s the only environment.

The node.js execution is currently only used for automated testing, but one can easily imagine a programming experience for scripts running on headless devices, either locally or in the cloud.

In case of microcontrollers, PXT programs are compiled in the browser to ARM Thumb assembly, and then to machine code, resulting in a file which is then deployed to the microcontroller, usually via USB mass-storage interface.

For browsers and node.js, PXT programs are compiled to continuation-passing style JavaScript. This utilizes the TypeScript abstract syntax tree as input, but does not use TypeScript JavaScript emitter. On the plus side, this allows for handling of async calls, even if the browser doesn’t support yield statement, as well as cross-browser and remote debugging. On the other hand, the generated code is not really human readable.

In the tagged strategy, numbers are either tagged 31-bit signed integers, or if they do not fit boxed doubles. Special constants like false, null and undefined are given special values and can be distinguished. We’re aiming at full JavaScript compatibility here.

Static compilation vs a dynamic VM

PXT programs are compiled to native code. The native targets include ARM Thumb, and an unfinished AVR port. The information below concerns the tagged compilation strategy.

Compared to a typical dynamic JavaScript engine, PXT compiles code statically, giving rise to significant time and space performance improvements:

  • user programs are compiled directly to machine code, and are never in any byte-code form that needs to be interpreted; this results in much faster execution than a typical JS interpreter
  • there is no RAM overhead for user-code - all code sits in flash; in a dynamic VM there are usually some data-structures representing code
  • due to lack of boxing for small integers and static class layout the memory consumption for objects is around half the one you get in a dynamic VM (not counting the user-code structures mentioned above)
  • while there is some runtime support code in PXT, it’s typically around 100KB smaller than a dynamic VM, bringing down flash consumption and leaving more space for user code

The execution time, RAM and flash consumption of PXT code is as a rule of thumb 2x of compiled C code, making it competitive to write drivers and other user-space libraries.

Interfacing C++ from PXT is easier than interfacing typical dynamic VMs, in particular for simple functions with numbers on input and output - there is no need for unboxing, checking types, or memory management.

The main disadvantage of using static compilation is lack of dynamic features in the language (think eval), as explained above.

While it is possible to run a dynamic VM even on an nRF51-class device (256KB of flash, 16KB of RAM), it leaves little space for innovative features on the software side, or more complex user programs and user-space (not C++) drivers.

Smaller int types

As noted above, when performing computations numbers are treated as doubles. However, when you store numbers in global variables or (soon) record fields you can choose to use a smaller int type to save memory. Microcontrollers typically have very little memory left, so these few bytes saved here and there (especially in commonly used packages) do add up.

The supported types are:

  • uint8 with range 0 to 255
  • uint16 with range 0 to 65536
  • int8 with range -128 to 127
  • int16 with range -32768 to 32767
  • int32 with range -2147483648 to 2147483647
  • uint32 with range 0 to 4294967295

If you attempt to store a number exceeding the range of the small int type, only the lowest 8 or 16 bits will be stored. There is no clamping nor overflow exceptions.

If you just use number type (or specify no type at all) in tagged strategy, then if the number fits in signed 31 bits, 4 bytes of memory will be used. Otherwise, the 4 bytes will point to a heap-allocated double (all together, with memory allocator overhead, around 20 bytes).

In legacy strategy, number is equivalent to int32, and there is no uint32.


  • arrays of int types are currently not supported; you can use a Buffer instead
  • locals and parameters of int types are not supported