JavaScript Support

As of revision 20509, KoLmafia supports scripting in JavaScript! You can run JS code from the CLI using js <code>, and you can call scripts through any of the normal methods. Consult and "lifecycle" scripts (e.g. betweenBattleScript) are supported as well as of revision 20519. This support is still experimental - you have been warned.

Basics

• All the methods in the ASH runtime library are available, with names of methods converted to camelCase. So, for example, print_html in ASH becomes printHtml in JS.
• In scripts called from a file, you can access the runtime library by calling require("kolmafia"), so const { printHtml } = require("kolmafia"). If running from the command line via js, the runtime library functions are all available in the global scope (so you can do js print("Hello world!");).
• ASH maps are converted to plain JS objects, and ASH arrays are converted to JS arrays.
• You can look at the type reference for the JS version of the ASH runtime library with jsref, which works just like ashref.
• Objects like monsters and items can be constructed via the Monster and Item global objects, along with the rest of ASH's enumerated types (stat, phylum, location, etc.). Monster.get takes a number or a string, just like $monster in ASH, or an array of numbers and strings to construct an array of monsters. Monster.all works like $monsters[] in ASH; it takes no arguments and returns an array of all monsters.
• Mafia supports require for both ASH and JS scripts. For ASH scripts, it will execute top-level code but only export functions, not variables, in the top scope.
• If you want Mafia to run your main function, you must export it by adding it to module.exports, just as you would for a Node module. For the uninitiated, this just means adding module.exports.main = main to the end of your script. You will want to do the same with any function or value you want to be available to other scripts via require.

JavaScript Version and Features

KoLmafia uses the Rhino engine to execute JavaScript code. Rhino supports an older version of JavaScript called "ES5", plus some features from newer versions. This means that many JavaScript features that work in web browsers might not work in KoLmafia.

Here is an incomplete list of post-ES5 features supported by Rhino (source):

Supported

• Syntax
  • let and (partially) const
    • Does not support block-level scoping or temporal dead zones, meaning that you cannot use const for loop variables. for (const a in obj) { ... } is a syntax error.
  • Array/object destructuring (but spread/rest syntax (...) is not supported)
  • for...of loop
  • Arrow functions: () => {}
  • Octal and binary literals
• Features
  • Symbol
  • Set, Map, WeakSet, WeakMap
  • ES2015 methods in Array, Math, Number, Object, String
  • Array.prototype.includes()
  • String.prototype.padStart()/padEnd()/trimStart()/trimEnd()
  • TypedArray: Can be constructed, but most TypedArray-specific methods are unavailable.

Unsupported

• Syntax
  • Spread/rest syntax (...)
  • Template string literals: Backtick string literals (``) are not a syntax error, but are treated as plain string literals.
  • Classes
  • ECMAScript modules (import/export)
  • Default function parameters
  • Computed property names
  • Async/Await
  • Trailing commas in function definitions (oddly, trailing commas are supported in function calls)
• Features
  • Promise
  • Proxy
  • Reflect

Other

Most JavaScript globals available in browsers and/or server-side environments like Node.js are not available. This includes alert(), console.log(), and setTimeout().

Transpiling

Folks with experience doing JavaScript web development are likely well-acquainted with tools such as Babel, Webpack, and TypeScript. These tools allow you to write modern JavaScript code. and can transpile the code down to an older version of JS supported by a particular engine. That approach works well with Rhino.

• Babel: As of r20558, you will still need to apply several patches to Babel in order to get babel-preset-env to work. See [1] for an example of a working Babel/Webpack/Typescript configuration; you'll need the configuration files and also the patches, which can be applied with patch-package.
• TypeScript: If you use Babel without TypeScript, setting the target to "ES5" will usually work.