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Minimal yet extensible Ruby to JavaScript conversion.

Build Status Gem Version


The base package maps Ruby syntax to JavaScript semantics. For example:

  • a Ruby Hash literal becomes a JavaScript Object literal
  • Ruby symbols become JavaScript strings.
  • Ruby method calls become JavaScript function calls IF there are either one or more arguments passed OR parenthesis are used
  • otherwise Ruby method calls become JavaScript property accesses.
  • by default, methods and procs return undefined
  • splats mapped to spread syntax when ES2015 or later is selected, and to equivalents using apply, concat, slice, and arguments otherwise.
  • ruby string interpolation is expanded into string + operations
  • and and or become && and ||
  • a ** b becomes Math.pow(a,b)
  • << a becomes .push(a)
  • unless becomes if !
  • until becomes while !
  • case and when becomes switch and case
  • ruby for loops become js for loops
  • (1...4).step(2){ becomes for (var i = 1; i < 4; i += 2) {
  • x.forEach { next } becomes x.forEach(function() {return})
  • lambda {} and proc {} becomes function() {}
  • class Person; end becomes function Person() {}
  • instance methods become prototype methods
  • instance variables become underscored, @name becomes this._name
  • self is assigned to this is if used
  • Any block becomes and explicit argument new Promise do; y(); end becomes new Promise(function() {y()})
  • regular expressions are mapped to js
  • raise becomes throw
  • expressions enclosed in backtick operators (``) and %x{} literals are evaluated in the context of the caller and the results are inserted into the generated JavaScript.

Ruby attribute accessors, methods defined with no parameters and no parenthesis, as well as setter method definitions, are mapped to Object.defineProperty, so avoid these if you wish to target users running IE8 or lower.

While both Ruby and JavaScript have open classes, Ruby unifies the syntax for defining and extending an existing class, whereas JavaScript does not. This means that Ruby2JS needs to be told when a class is being extended, which is done by prepending the class keyword with two plus signs, thus: ++class C; ...; end.

Filters may be provided to add Ruby-specific or framework specific behavior. Filters are essentially macro facilities that operate on an AST representation of the code.

See notimplemented_spec for a list of Ruby features known to be not implemented.



require 'ruby2js'
puts Ruby2JS.convert("a={age:3}\na.age+=1")

With filter:

require 'ruby2js/filter/functions'
puts Ruby2JS.convert('"2A".to_i(16)')

Enable ES2015 support:

puts Ruby2JS.convert('"#{a}"', eslevel: 2015)

Enable strict support:

puts Ruby2JS.convert('a=1', strict: true)

Emit strict equality comparisons:

puts Ruby2JS.convert('a==1', comparison: :identity)

With ExecJS:

require 'ruby2js/execjs'
require 'date'

context = Ruby2JS.compile('d = new Date(%Y, %-m-1, %-d)'))
puts context.eval('d.getYear()')+1900

Conversions can be explored interactively using the demo provided.


JavaScript is a language where 0 is considered false, strings are immutable, and the behaviors for operators like == are, at best, convoluted.

Any attempt to bridge the semantics of Ruby and JavaScript will involve trade-offs. Consider the following expression:


Programmers who are familiar with Ruby will recognize that this returns the last element (or character) of an array (or string). However, the meaning is quite different if a is a Hash.

One way to resolve this is to change the way indexing operators are evaluated, and to provide a runtime library that adds properties to global JavaScript objects to handle this. This is the approach that Opal takes. It is a fine approach, with a number of benefits. It also has some notable drawbacks. For example, readability and compatibility with other frameworks.

Another approach is to simply accept JavaScript semantics for what they are. This would mean that negative indexes would return undefined for arrays and strings. This is the base approach provided by ruby2js.

A third approach would be to do static transformations on the source in order to address common usage patterns or idioms. These transformations can even be occasionally unsafe, as long as the transformations themselves are opt-in. ruby2js provides a number of such filters, including one that handles negative indexes when passed as a literal. As indicated above, this is unsafe in that it will do the wrong thing when it encounters a hash index which is expressed as a literal constant negative one. My experience is that such is rare enough to be safely ignored, but YMMV. More troublesome, this also won’t work when the index is not a literal (e.g., a[n]) and the index happens to be negative at runtime.

This quickly gets into gray areas. each in Ruby is a common method that facilitates iteration over arrays. forEach is the JavaScript equivalent. Mapping this is fine until you start using a framework like jQuery which provides a function named each.

Fortunately, Ruby provides ? and ! as legal suffixes for method names, Ruby2js filters do an exact match, so if you select a filter that maps each to forEach, each! will pass through the filter. The final code that emits JavaScript function calls and parameter accesses will strip off these suffixes.

This approach works well if it is an occasional change, but if the usage is pervasive, most filters support options to exclude a list of mappings, for example:

puts Ruby2JS.convert('jQuery("li").each {|index| ...}', exclude: :each)

Alternatively, you can change the default:

Ruby2JS::Filter.exclude :each

Static transformations and runtime libraries aren't aren’t mutually exclusive. With enough of each, one could reproduce any functionality desired. Just be forewarned, that implementing a function like method_missing would require a lot of work.


While this is a low level library suitable for DIY integration, one of the obvious uses of a tool that produces JavaScript is by web servers. Ruby2JS includes three such integrations:

As you might expect, CGI is a bit sluggish. By contrast, Sinatra and Rails are quite speedy as the bulk of the time is spent on the initial load of the required libraries.


In general, making use of a filter is as simple as requiring it. If multiple filters are selected, they will all be applied in parallel in one pass through the script.

  • return adds return to the last expression in functions.

  • require supports require and require_relative statements. Contents of files that are required are converted to JavaScript and expanded inline. require function calls in expressions are left alone.

  • camelCase converts underscore_case to camelCase. See camelCase_spec for examples.

  • functions

    • .all? becomes .every
    • .any? becomes .some
    • .chr becomes fromCharCode
    • .clear becomes .length = 0
    • .delete becomes delete target[arg]
    • .downcase becomes .toLowerCase
    • .each becomes .forEach
    • .each_key becomes for (i in ...) {}
    • .each_pair becomes for (var key in item) {var value = item[key]; ...}
    • .each_value becomes .forEach
    • .each_with_index becomes .forEach
    • .end_with? becomes .slice(-arg.length) == arg
    • .empty? becomes .length == 0
    • .find_index becomes findIndex
    • .first becomes [0]
    • .first(n) becomes .slice(0, n)
    • .gsub becomes replace(//g)
    • .include? becomes .indexOf() != -1
    • .inspect becomes JSON.stringify()
    • .keys() becomes Object.keys()
    • .last becomes [*.length-1]
    • .last(n) becomes .slice(*.length-1, *.length)
    • .lstrip becomes .replace(/^\s+/, "")
    • .max becomes Math.max.apply(Math)
    • .merge becomes Object.assign({}, ...)
    • .merge! becomes Object.assign()
    • .min becomes Math.min.apply(Math)
    • .nil? becomes == null
    • .ord becomes charCodeAt(0)
    • puts becomes console.log
    • .replace becomes .length = 0; ...push.apply(*)
    • .respond_to? becomes right in left
    • .rstrip becomes .replace(/s+$/, "")
    • .scan becomes .match(//g)
    • .start_with? becomes .substring(0, arg.length) == arg
    • .upto(lim) becomes for (var i=num; i<=lim; i+=1)
    • .downto(lim) becomes for (var i=num; i>=lim; i-=1)
    • .step(lim, n).each becomes for (var i=num; i<=lim; i+=n)
    • .step(lim, -n).each becomes for (var i=num; i>=lim; i-=n)
    • (0..a).to_a becomes Array.apply(null, {length: a}).map(, Number)
    • (b..a).to_a becomes Array.apply(null, {length: (a-b+1)}).map(, Number).map(function (idx) { return idx+b })
    • (b...a).to_a becomes Array.apply(null, {length: (a-b)}).map(, Number).map(function (idx) { return idx+b })
    • .strip becomes .trim
    • .sub becomes .replace
    • .to_f becomes parseFloat
    • .to_i becomes parseInt
    • .to_s becomes .to_String
    • .upcase becomes .toUpperCase
    • [-n] becomes [*.length-n] for literal values of n
    • [n...m] becomes .slice(n,m)
    • [n..m] becomes .slice(n,m+1)
    • [/r/, n] becomes .match(/r/)[n]
    • [/r/, n]= becomes .replace(/r/, ...)
    • (1..2).each {|i| ...} becomes for (var i=1 i<=2; i+=1)
    • "string" * length becomes new Array(length + 1).join("string")
    • .sub! and .gsub! become equivalent x = x.replace statements
    • .map!, .reverse!, and .select become equivalent .splice(0, .length, *.method()) statements
    • becomes this._foo(args)
    • becomes this.constructor._foo(args)
    • Array(x) becomes
    • delete x becomes delete x (note lack of parenthesis)
    • setInterval and setTimeout allow block to be treated as the first parameter on the call
    • for the following methods, if the block consists entirely of a simple expression (or ends with one), a return is added prior to the expression: sub, gsub, any?, all?, map, find, find_index.
    • New classes subclassed off of Exception will become subclassed off of Error instead; and default constructors will be provided
    • loop do...end will be replaced with while (true) {...}
    • raise will be replaced with throw new Error(...)

    Additionally, there is one mapping that will only be done if explicitly included:

    • .class becomes .constructor
  • node

    • `command` becomes child_process.execSync("command", {encoding: "utf8"})
    • ARGV becomes process.argv.slice(2)
    • __dir__ becomes __dirname
    • Dir.chdir becomes process.chdir
    • Dir.entries becomes fs.readdirSync
    • Dir.mkdir becomes fs.mkdirSync
    • Dir.mktmpdir becomes fs.mkdtempSync
    • Dir.pwd becomes process.cwd
    • Dir.rmdir becomes fs.rmdirSync
    • ENV becomes process.env
    • __FILE__ becomes __filename
    • File.chmod becomes fs.chmodSync
    • File.chown becomes fs.chownSync
    • File.cp becomes fs.copyFileSync
    • File.exist? becomes fs.existsSync
    • File.lchmod becomes fs.lchmodSync
    • becomes fs.linkSync
    • File.ln becomes fs.linkSync
    • File.lstat becomes fs.lstatSync
    • becomes fs.readFileSync
    • File.readlink becomes fs.readlinkSync
    • File.realpath becomes fs.realpathSync
    • File.rename becomes fs.renameSync
    • File.stat becomes fs.statSync
    • File.symlink becomes fs.symlinkSync
    • File.truncate becomes fs.truncateSync
    • File.unlink becomes fs.unlinkSync
    • becomes process.chdir
    • FileUtils.cp becomes fs.copyFileSync
    • FileUtils.ln becomes fs.linkSync
    • FileUtils.ln_s becomes fs.symlinkSync
    • FileUtils.mkdir becomes fs.mkdirSync
    • becomes fs.renameSync
    • FileUtils.pwd becomes process.cwd
    • FileUtils.rm becomes fs.unlinkSync
    • becomes fs.readFileSync
    • IO.write becomes fs.writeFileSync
    • system becomes child_process.execSync(..., {stdio: "inherit"})
  • nokogiri

    • add_child becomes appendChild
    • add_next_sibling becomes node.parentNode.insertBefore(sibling, node.nextSibling)
    • add_previous_sibling becomes node.parentNode.insertBefore(sibling, node)
    • after becomes node.parentNode.insertBefore(sibling, node.nextSibling)
    • at becomes querySelector
    • attr becomes getAttribute
    • attribute becomes getAttributeNode
    • before becomes node.parentNode.insertBefore(sibling, node)
    • cdata? becomes node.nodeType === Node.CDATA_SECTION_NODE
    • children becomes childNodes
    • comment? becomes node.nodeType === Node.COMMENT_NODE
    • content becomes textContent
    • create_element becomes createElement
    • document becomes ownerDocument
    • element? becomes node.nodeType === Node.ELEMENT_NODE
    • fragment? becomes node.nodeType === Node.FRAGMENT_NODE
    • get_attribute becomes getAttribute
    • has_attribute becomes hasAttribute
    • inner_html becomes innerHTML
    • key? becomes hasAttribute
    • name becomes nextSibling
    • next becomes nodeName
    • next= becomes node.parentNode.insertBefore(sibling,node.nextSibling)
    • next_element becomes nextElement
    • next_sibling becomes nextSibling
    • Nokogiri::HTML5 becomes new JSDOM().window.document
    • Nokogiri::HTML5.parse becomes new JSDOM().window.document
    • Nokogiri::HTML becomes new JSDOM().window.document
    • Nokogiri::HTML.parse becomes new JSDOM().window.document
    • becomes document.createElement()
    • parent becomes parentNode
    • previous= becomes node.parentNode.insertBefore(sibling, node)
    • previous_element becomes previousElement
    • previous_sibling becomes previousSibling
    • processing_instruction? becomes node.nodeType === Node.PROCESSING_INSTRUCTION_NODE
    • remove_attribute becomes removeAttribute
    • root becomes documentElement
    • search becomes querySelectorAll
    • set_attribute becomes setAttribute
    • text? becomes node.nodeType === Node.TEXT_NODE
    • text becomes textContent
    • to_html becomes outerHTML
  • rubyjs

    • .at() becomes
    • .between?() becomes R().between()
    • .capitalize() becomes _s.capitalize()
    • .center() becomes
    • .chomp() becomes _s.chomp()
    • .collect_concat() becomes _e.collect_concat()
    • .compact() becomes _a.compact()
    • .compact!() becomes _a.compact_bang()
    • .count() becomes _e.count()
    • .cycle() becomes _e.cycle()
    • .delete_at() becomes _a.delete_at()
    • .delete_if() becomes _a.delete_if()
    • .drop_while() becomes _e.drop_while()
    • .each_index() becomes _e.each_index()
    • .each_slice() becomes _e.each_slice()
    • .each_with_index() becomes _e.each_with_index()
    • .each_with_object() becomes _e.each_with_object()
    • .find_all() becomes _e.find_all()
    • .find() becomes _e.find()
    • .flat_map() becomes _e.flat_map()
    • .flatten() becomes _a.flatten()
    • .grep() becomes _e.grep()
    • .group_by() becomes _e.group_by()
    • .inject() becomes _e.inject()
    • .insert() becomes _a.insert()
    • .keep_if() becomes _a.keep_if()
    • .ljust() becomes _s.ljust()
    • .lstrip() becomes _s.lstrip()
    • .map() becomes
    • .max_by() becomes _e.max_by()
    • .min_by() becomes _e.min_by()
    • .one?() becomes
    • .partition() becomes _e.partition()
    • .reject() becomes _e.reject()
    • .reverse() becomes _a.reverse()
    • .reverse!() becomes _a.reverse_bang()
    • .reverse_each() becomes _e.reverse_each()
    • .rindex() becomes _s.rindex()
    • .rjust() becomes _s.rjust()
    • .rotate() becomes _a.rotate()
    • .rotate!() becomes _a.rotate_bang()
    • .rstrip() becomes _s.rstrip()
    • .scan() becomes _s.scan()
    • .select() becomes
    • .shift() becomes _a.shift()
    • .shuffle() becomes _a.shuffle()
    • .shuffle!() becomes _a.shuffle_bang()
    • .slice() becomes _a.slice()
    • .slice!() becomes _a.slice_bang()
    • .sort_by() becomes _e.sort_by()
    • .strftime() becomes _t.strftime()
    • .swapcase() becomes _s.swapcase()
    • .take_while() becomes _e.take_while()
    • .transpose() becomes _a.transpose()
    • .tr() becomes
    • .union() becomes _a.union()
    • .uniq() becomes _a.uniq()
    • .uniq!() becomes _a.uniq_bang()
    • <=> becomes R.Comparable.cmp()
    • (n..m) becomes
  • underscore

    • .clone() becomes _.clone()
    • .compact() becomes _.compact()
    • .count_by {} becomes _.countBy {}
    • .find {} becomes _.find {}
    • .find_by() becomes _.findWhere()
    • .flatten() becomes _.flatten()
    • .group_by {} becomes _.groupBy {}
    • .has_key?() becomes _.has()
    • .index_by {} becomes _.indexBy {}
    • .invert() becomes _.invert()
    • .invoke(&:n) becomes _.invoke(, :n)
    • .map(&:n) becomes _.pluck(, :n)
    • .merge!() becomes _.extend()
    • .merge() becomes _.extend({}, )
    • .reduce {} becomes _.reduce {}
    • .reduce() becomes _.reduce()
    • .reject {} becomes _.reject {}
    • .sample() becomes _.sample()
    • .select {} becomes {}
    • .shuffle() becomes _.shuffle()
    • .size() becomes _.size()
    • .sort() becomes _.sort_by(, _.identity)
    • .sort_by {} becomes _.sortBy {}
    • .times {} becomes _.times {}
    • .values() becomes _.values()
    • .where() becomes _.where()
    • .zip() becomes
    • (n...m) becomes _.range(n, m)
    • (n..m) becomes _.range(n, m+1)
    • .compact!, .flatten!, shuffle!, reject!, sort_by!, and .uniq become equivalent .splice(0, .length, *.method()) statements
    • for the following methods, if the block consists entirely of a simple expression (or ends with one), a return is added prior to the expression: reduce, sort_by, group_by, index_by, count_by, find, select, reject.
    • is_a? and kind_of? map to === "[object #{type}]" for the following types: Arguments, Boolean, Date, Error, Function, Number, Object, RegExp, String; and maps Ruby names to JavaScript equivalents for Exception, Float, Hash, Proc, and Regexp. Additionally, is_a?andkind_of?map toArray.isArray()forArray`.
  • jquery

    • maps Ruby unary operator ~ to jQuery $ function
    • maps Ruby attribute syntax to jquery attribute syntax
    • .to_a becomes toArray
    • maps $$ to jQuery $ function
    • defaults the fourth parameter of $$.post to "json", allowing Ruby block syntax to be used for the success function.
  • minitest-jasmine

    • maps subclasses of Minitest::Test to describe calls
    • maps test_ methods inside subclasses of Minitest::Test to it calls
    • maps setup, teardown, before, and after calls to beforeEach and afterEach calls
    • maps assert and refute calls to expect...toBeTruthy() and toBeFalsy calls
    • maps assert_equal, refute_equal, .must_equal and .cant_equal calls to expect...toBe() calls
    • maps assert_in_delta, refute_in_delta, .must_be_within_delta, .must_be_close_to, .cant_be_within_delta, and .cant_be_close_to calls to expect...toBeCloseTo() calls
    • maps assert_includes, refute_includes, .must_include, and .cant_include calls to expect...toContain() calls
    • maps assert_match, refute_match, .must_match, and .cant_match calls to expect...toMatch() calls
    • maps assert_nil, refute_nil, .must_be_nil, and .cant_be_nill calls to expect...toBeNull() calls
    • maps assert_operator, refute_operator, .must_be, and .cant_be calls to expect...toBeGreaterThan() or toBeLessThan calls
  • cjs

    • maps export def f to exports.f =
    • maps export async def f to exports.f = async
    • maps export v = to exports.v =
    • maps export default proc to module.exports =
    • maps export default async proc to module.exports = async
    • maps export default to module.exports =
  • matchAll

    For ES level < 2020:

    • maps str.matchAll(pattern).forEach {} to while (match = pattern.exec(str)) {}

    Note pattern must be a simple variable with a value of a regular expression with the g flag set at runtime.

Wunderbar includes additional demos:

ES2015 support

When option eslevel: 2015 is provided, the following additional conversions are made:

  • "#{a}" becomes `${a}`
  • a = 1 becomes let a = 1
  • A = 1 becomes const A = 1
  • a, b = b, a becomes [a, b] = [b, a]
  • a, (foo, *bar) = x becomes let [a, [foo,]] = x
  • def f(a, (foo, *bar)) becomes function f(a, [foo,])
  • def a(b=1) becomes function a(b=1)
  • def a(*b) becomes function a(...b)
  • .each_value becomes for (i of ...) {}
  • a(*b) becomes a(...b)
  • "#{a}" becomes `${a}`
  • lambda {|x| x} becomes (x) => {return x}
  • proc {|x| x} becomes (x) => {x}
  • a {|x|} becomes a((x) => {})
  • class Person; end becomes class Person {}
  • (0...a).to_a becomes [...Array(a).keys()]
  • (0..a).to_a becomes [...Array(a+1).keys()]
  • (b..a).to_a becomes Array.from({length: (a-b+1)}, (_, idx) => idx+b)

ES2015 class support includes constructors, super, methods, class methods, instance methods, instance variables, class variables, getters, setters, attr_accessor, attr_reader, attr_writer, etc.

Additionally, the functions filter will provide the following conversion:

  • Array(x) becomes Array.from(x)
  • .inject(n) {} becomes .reduce(() => {}, n)

Finally, keyword arguments and optional keyword arguments will be mapped to parameter detructuring.

ES2016 support

When option eslevel: 2016 is provided, the following additional conversion is made:

  • a ** b becomes a ** b
  • .include? becomes .includes

ES2017 support

When option eslevel: 2017 is provided, the following additional conversions are made by the functions filter:

  • .values() becomes Object.values()
  • .entries() becomes Object.entries()
  • .each_pair {} becomes `for (let [key, value] of Object.entries()) {}'

ES2018 support

When option eslevel: 2018 is provided, the following additional conversion is made by the functions filter:

  • .merge becomes {...a, ...b}

Additionally, rest arguments can now be used with keyword arguments and optional keyword arguments.

ES2019 support

When option eslevel: 2019 is provided, the following additional conversion is made by the functions filter:

  • .flatten becomes .flat(Infinity)
  • .lstrip becomes `.trimEnd
  • .rstrip becomes `.trimStart
  • a.to_h becomes Object.fromEntries(a)
  • Hash[a] becomes Object.fromEntries(a)

Additionally, rescue without a variable will map to catch without a variable.

ES2020 support

When option eslevel: 2020 is provided, the following additional conversions are made:

  • @x becomes this.#x
  • @@x becomes ClassName.#x
  • a&.b becomes a?.b
  • .scan becomes Array.from(str.matchAll(/.../g), s => s.slice(1))

Picking a Ruby to JS mapping tool

dsl — A domain specific language, where code is written in one language and errors are given in another. -- Devil’s Dictionary of Programming

If you simply want to get a job done, and would like a mature and tested framework, and only use one of the many integrations that Opal provides, then Opal is the way to go right now.

ruby2js is for those that want to produce JavaScript that looks like it wasn’t machine generated, and want the absolute bare minimum in terms of limitations as to what JavaScript can be produced.

Try for yourself. Compare.

And, of course, the right solution might be to use CoffeeScript instead.


(The MIT License)

Copyright (c) 2009, 2013 Macario Ortega, Sam Ruby

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.



Ruby to Javascript conversion



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