pxt-calliope/docs/javascript.md

JavaScript

We support a "static" subset of TypeScript (itself a superset of JavaScript):

• Calling
• Sequencing
• Variables
• Operators
• Statements
• Functions
• Types
• Classes

Supported language features

• variables with let and 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 and for ... of
• break/continue; also with labeled loops
• switch statement (on numbers only)
• 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 () => ...
• classes with fields, methods and constructors; new keyword
• array literals [1, 2, 3]
• enums
• class inheritance

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 ... in statements
• object literals { foo: 1, bar: "two" }
• method-like properties (get/set accessors)
• debugger statement for breakpoints

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)