JavaScript Support

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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.


  • 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!");).
    • If you want all runtime functions available, you can use const k = require("kolmafia"), and then call functions using k.printHtml("Hello world!"); (as an example)
  • 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.
  • 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.

Data Type Classes

All enumerated data types in ASH are available as classes. For example, monster is available as the Monster class, and item is available as the Item class.

Each enumerated class has the following static methods:

  • <ClassName>.get() takes a number or string and returns an object of the class.
    • For example, Monster.get("fiendish can of asparagus") is equivalent to $monster[ fiendish can of asparagus ] in ASH. Item.get(1) is equivalent to $item[ 1 ].
    • This also accepts string representations of integers. For example, Item.get(5) and Item.get("5") return the same result.
  • <ClassName>.get() can also take an array of numbers and strings, and return an array of objects.
    • For example, Item.get(["seal-clubbing club", "pail", 5]) is similar to $items[ seal-clubbing club, pail, 5 ] in ASH. However, the JavaScript version returns an array of objects, instead of a boolean map.
    • Passing an empty array returns an empty array (i.e. Item.get([]) is not the same as $items[]).
  • <ClassName>.all() takes no argument and returns all possible values of the class.
    • For example, Monster.all() returns an array of all known monsters. This is similar to $monsters[] in ASH.

Enumerated objects support the toString() method, which acts like the to_string() ASH function.

Note: As of r20620, KoLmafia no longer provides a custom valueOf() method for enumerated objects. This means that you can no longer retrieve the ID of an item, effect, or familiar using Number(). Instead, use the toInt() library function.


let item = Item.get("filthy lucre");
let itemId = toInt(item);

ASH and JavaScript Interoperability

JavaScript scripts can require() ASH scripts and use their functions. When an ASH script is require()-ed by JavaScript code, KoLmafia will execute top-level code, but only export functions in the top-level scope. ASH variables are not exported.

For example, you can use Zlib if it is installed:

1 const { getvar } = require("zlib.ash");
2 let myvar = getvar("SOME_VAR_NAME");

ASH scripts cannot import JavaScript scripts.

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):


  • 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.


  • 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


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


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.


If you use TypeScript without Babel, you can set the target to "ES5".

TypeScript does not provide polyfills for modern JavaScript APIs missing in Rhino. To use them, you must supply your own.

To avoid accidentally using any missing APIs, we recommend using the following configuration for your tsconfig.json:

  "compilerOptions": {
    // JavaScript APIs supported by Rhino 1.7.13
    // See for more info
    "lib": [
    // Rhino uses require() instead of import/export
    // Note: If you use Webpack or Rollup, change this to "ES2015"
    "module": "CommonJS",
    // Rhino supports ES5+
    // Note: If you use Babel with TypeScript, change this to "ESNext"
    "target": "ES5"

If you use a bundler such as Webpack or Rollup, you should change the module to "ES2015", and let the bundlers convert your code to CommonJS.