Using dart to create a javascript library

Question

The problem

I'm currently working on a JavaScript library, and in order to reduce the amount of bugs I thought that my library might benefit from using Dart's static typing mechanism. First, because my lib wasn't doing any interop neither with HTML nor with other JavaScript libraries, only pure javascript object manipulation stuff. However I didn't find any info on the net showing how it is possible to build a JS library using dart. So I've tried to do that myself, created initial dart file:

library Repo;

class Type {
  final String name;
  final TypeCategory category;
  
  Type(String name, TypeCategory category) : name = name, category = category {
    category.types[name] = this;
  }
}

class TypeCategory {
  final String name;
  final Map<String, Type> types = new Map();
  
  TypeCategory(this.name);
}

class Branch {
  
}

class Descriptor {
  
}

class TableDescriptor extends Descriptor {
  TableDescriptor.ctor() {
    
  }
}

class Repo {
  Descriptor descriptor(String name) {
    
  }
  
  Branch branch(String name) {
    
  }
  
  void Merge() {
    
  }
}


main() {
  return Repo;
}

Compiled it to JavaScript using dart2js to see how I'm doing:

// Generated by dart2js, the Dart to JavaScript compiler version: 1.3.6.
// The code supports the following hooks:
// dartPrint(message):
//    if this function is defined it is called instead of the Dart [print]
//    method.
//
// dartMainRunner(main, args):
//    if this function is defined, the Dart [main] method will not be invoked
//    directly. Instead, a closure that will invoke [main], and its arguments
//    [args] is passed to [dartMainRunner].
(function($) {
function dart(){ this.x = 0 }var A = new dart;
delete A.x;
var B = new dart;
delete B.x;
var C = new dart;
delete C.x;
var D = new dart;
delete D.x;
var E = new dart;
delete E.x;
var F = new dart;
delete F.x;
var G = new dart;
delete G.x;
var H = new dart;
delete H.x;
var J = new dart;
delete J.x;
var K = new dart;
delete K.x;
var L = new dart;
delete L.x;
var M = new dart;
delete M.x;
var N = new dart;
delete N.x;
var O = new dart;
delete O.x;
var P = new dart;
delete P.x;
var Q = new dart;
delete Q.x;
var R = new dart;
delete R.x;
var S = new dart;
delete S.x;
var T = new dart;
delete T.x;
var U = new dart;
delete U.x;
var V = new dart;
delete V.x;
var W = new dart;
delete W.x;
var X = new dart;
delete X.x;
var Y = new dart;
delete Y.x;
var Z = new dart;
delete Z.x;
function Isolate() {}
init();

$ = Isolate.$isolateProperties;
var $$ = {};

(function (reflectionData) {
  "use strict";
  function map(x){x={x:x};delete x.x;return x}
    function processStatics(descriptor) {
      for (var property in descriptor) {
        if (!hasOwnProperty.call(descriptor, property)) continue;
        if (property === "^") continue;
        var element = descriptor[property];
        var firstChar = property.substring(0, 1);
        var previousProperty;
        if (firstChar === "+") {
          mangledGlobalNames[previousProperty] = property.substring(1);
          var flag = descriptor[property];
          if (flag > 0) descriptor[previousProperty].$reflectable = flag;
          if (element && element.length) init.typeInformation[previousProperty] = element;
        } else if (firstChar === "@") {
          property = property.substring(1);
          $[property]["@"] = element;
        } else if (firstChar === "*") {
          globalObject[previousProperty].$defaultValues = element;
          var optionalMethods = descriptor.$methodsWithOptionalArguments;
          if (!optionalMethods) {
            descriptor.$methodsWithOptionalArguments = optionalMethods = {}
          }
          optionalMethods[property] = previousProperty;
        } else if (typeof element === "function") {
          globalObject[previousProperty = property] = element;
          functions.push(property);
          init.globalFunctions[property] = element;
        } else if (element.constructor === Array) {
          addStubs(globalObject, element, property, true, descriptor, functions);
        } else {
          previousProperty = property;
          var newDesc = {};
          var previousProp;
          for (var prop in element) {
            if (!hasOwnProperty.call(element, prop)) continue;
            firstChar = prop.substring(0, 1);
            if (prop === "static") {
              processStatics(init.statics[property] = element[prop]);
            } else if (firstChar === "+") {
              mangledNames[previousProp] = prop.substring(1);
              var flag = element[prop];
              if (flag > 0) element[previousProp].$reflectable = flag;
            } else if (firstChar === "@" && prop !== "@") {
              newDesc[prop.substring(1)]["@"] = element[prop];
            } else if (firstChar === "*") {
              newDesc[previousProp].$defaultValues = element[prop];
              var optionalMethods = newDesc.$methodsWithOptionalArguments;
              if (!optionalMethods) {
                newDesc.$methodsWithOptionalArguments = optionalMethods={}
              }
              optionalMethods[prop] = previousProp;
            } else {
              var elem = element[prop];
              if (prop !== "^" && elem != null && elem.constructor === Array && prop !== "<>") {
                addStubs(newDesc, elem, prop, false, element, []);
              } else {
                newDesc[previousProp = prop] = elem;
              }
            }
          }
          $$[property] = [globalObject, newDesc];
          classes.push(property);
        }
      }
    }
  function addStubs(descriptor, array, name, isStatic, originalDescriptor, functions) {
    var f, funcs = [originalDescriptor[name] = descriptor[name] = f = array[0]];
    f.$stubName = name;
    functions.push(name);
    for (var index = 0; index < array.length; index += 2) {
      f = array[index + 1];
      if (typeof f != "function") break;
      f.$stubName = array[index + 2];
      funcs.push(f);
      if (f.$stubName) {
        originalDescriptor[f.$stubName] = descriptor[f.$stubName] = f;
        functions.push(f.$stubName);
      }
    }
    for (var i = 0; i < funcs.length; index++, i++) {
      funcs[i].$callName = array[index + 1];
    }
    var getterStubName = array[++index];
    array = array.slice(++index);
    var requiredParameterInfo = array[0];
    var requiredParameterCount = requiredParameterInfo >> 1;
    var isAccessor = (requiredParameterInfo & 1) === 1;
    var isSetter = requiredParameterInfo === 3;
    var isGetter = requiredParameterInfo === 1;
    var optionalParameterInfo = array[1];
    var optionalParameterCount = optionalParameterInfo >> 1;
    var optionalParametersAreNamed = (optionalParameterInfo & 1) === 1;
    var isIntercepted = requiredParameterCount + optionalParameterCount != funcs[0].length;
    var functionTypeIndex = array[2];
    var unmangledNameIndex =  2 * optionalParameterCount + requiredParameterCount + 3;
    var isReflectable = array.length > unmangledNameIndex;

    if (getterStubName) {
      f = tearOff(funcs, array, isStatic, name, isIntercepted);
      f.getterStub = true;
      if (isStatic) init.globalFunctions[name] = f;
      originalDescriptor[getterStubName] = descriptor[getterStubName] = f;
      funcs.push(f);
      if (getterStubName) functions.push(getterStubName);
      f.$stubName = getterStubName;
      f.$callName = null;
      if (isIntercepted) init.interceptedNames[getterStubName] = true;
    }
    if (isReflectable) {
      for (var i = 0; i < funcs.length; i++) {
        funcs[i].$reflectable = 1;
        funcs[i].$reflectionInfo = array;
      }
      var mangledNames = isStatic ? init.mangledGlobalNames : init.mangledNames;
      var unmangledName = array[unmangledNameIndex];
      var reflectionName = unmangledName;
      if (getterStubName) mangledNames[getterStubName] = reflectionName;
      if (isSetter) {
        reflectionName += "=";
      } else if (!isGetter) {
        reflectionName += ":" + requiredParameterCount + ":" + optionalParameterCount;
      }
      mangledNames[name] = reflectionName;
      funcs[0].$reflectionName = reflectionName;
      funcs[0].$metadataIndex = unmangledNameIndex + 1;
      if (optionalParameterCount) descriptor[unmangledName + "*"] = funcs[0];
    }
  }
  function tearOffGetterNoCsp(funcs, reflectionInfo, name, isIntercepted) {
    return isIntercepted
        ? new Function("funcs", "reflectionInfo", "name", "H", "c",
            "return function tearOff_" + name + (functionCounter++)+ "(x) {" +
              "if (c === null) c = H.closureFromTearOff(" +
                  "this, funcs, reflectionInfo, false, [x], name);" +
              "return new c(this, funcs[0], x, name);" +
            "}")(funcs, reflectionInfo, name, H, null)
        : new Function("funcs", "reflectionInfo", "name", "H", "c",
            "return function tearOff_" + name + (functionCounter++)+ "() {" +
              "if (c === null) c = H.closureFromTearOff(" +
                  "this, funcs, reflectionInfo, false, [], name);" +
              "return new c(this, funcs[0], null, name);" +
            "}")(funcs, reflectionInfo, name, H, null)
  }
  function tearOffGetterCsp(funcs, reflectionInfo, name, isIntercepted) {
    var cache = null;
    return isIntercepted
        ? function(x) {
            if (cache === null) cache = H.closureFromTearOff(this, funcs, reflectionInfo, false, [x], name);
            return new cache(this, funcs[0], x, name)
          }
        : function() {
            if (cache === null) cache = H.closureFromTearOff(this, funcs, reflectionInfo, false, [], name);
            return new cache(this, funcs[0], null, name)
          }
  }
  function tearOff(funcs, reflectionInfo, isStatic, name, isIntercepted) {
    var cache;
    return isStatic
        ? function() {
            if (cache === void 0) cache = H.closureFromTearOff(this, funcs, reflectionInfo, true, [], name).prototype;
            return cache;
          }
        : tearOffGetter(funcs, reflectionInfo, name, isIntercepted);
  }
  var functionCounter = 0;
  var tearOffGetter = (typeof dart_precompiled == "function")
      ? tearOffGetterCsp : tearOffGetterNoCsp;
  if (!init.libraries) init.libraries = [];
  if (!init.mangledNames) init.mangledNames = map();
  if (!init.mangledGlobalNames) init.mangledGlobalNames = map();
  if (!init.statics) init.statics = map();
  if (!init.typeInformation) init.typeInformation = map();
  if (!init.globalFunctions) init.globalFunctions = map();
  if (!init.interceptedNames) init.interceptedNames = map();
  var libraries = init.libraries;
  var mangledNames = init.mangledNames;
  var mangledGlobalNames = init.mangledGlobalNames;
  var hasOwnProperty = Object.prototype.hasOwnProperty;
  var length = reflectionData.length;
  for (var i = 0; i < length; i++) {
    var data = reflectionData[i];
    var name = data[0];
    var uri = data[1];
    var metadata = data[2];
    var globalObject = data[3];
    var descriptor = data[4];
    var isRoot = !!data[5];
    var fields = descriptor && descriptor["^"];
    var classes = [];
    var functions = [];
    processStatics(descriptor);
    libraries.push([name, uri, classes, functions, metadata, fields, isRoot,
                    globalObject]);
  }
})
([
["Repo", "repo.dart", , F, {
  "^": "",
  main: function() {
    return C.Type_Jeh;
  }
},
1],
["_js_helper", "dart:_js_helper", , H, {
  "^": "",
  createRuntimeType: function($name) {
    return new H.TypeImpl($name, null);
  },
  TypeImpl: {
    "^": "Object;_typeName,_unmangledName"
  }
}],
["dart.core", "dart:core", , P, {
  "^": "",
  Null: {
    "^": "Object;"
  },
  Object: {
    "^": ";"
  }
}],
]);
Isolate.$finishClasses($$, $, null);
$$ = null;

// Runtime type support
// getInterceptor methods
C.Type_Jeh = H.createRuntimeType('Repo');
$.libraries_to_load = {};
$.Closure_functionCounter = 0;
$.BoundClosure_selfFieldNameCache = null;
$.BoundClosure_receiverFieldNameCache = null;

init.functionAliases = {};
;
init.metadata = [];
$ = null;
Isolate = Isolate.$finishIsolateConstructor(Isolate);
$ = new Isolate();
function convertToFastObject(properties) {
  function MyClass() {};
  MyClass.prototype = properties;
  new MyClass();
  return properties;
}
A = convertToFastObject(A);
B = convertToFastObject(B);
C = convertToFastObject(C);
D = convertToFastObject(D);
E = convertToFastObject(E);
F = convertToFastObject(F);
G = convertToFastObject(G);
H = convertToFastObject(H);
J = convertToFastObject(J);
K = convertToFastObject(K);
L = convertToFastObject(L);
M = convertToFastObject(M);
N = convertToFastObject(N);
O = convertToFastObject(O);
P = convertToFastObject(P);
Q = convertToFastObject(Q);
R = convertToFastObject(R);
S = convertToFastObject(S);
T = convertToFastObject(T);
U = convertToFastObject(U);
V = convertToFastObject(V);
W = convertToFastObject(W);
X = convertToFastObject(X);
Y = convertToFastObject(Y);
Z = convertToFastObject(Z);
// BEGIN invoke [main].
;(function (callback) {
  if (typeof document === "undefined") {
    callback(null);
    return;
  }
  if (document.currentScript) {
    callback(document.currentScript);
    return;
  }

  var scripts = document.scripts;
  function onLoad(event) {
    for (var i = 0; i < scripts.length; ++i) {
      scripts[i].removeEventListener("load", onLoad, false);
    }
    callback(event.target);
  }
  for (var i = 0; i < scripts.length; ++i) {
    scripts[i].addEventListener("load", onLoad, false);
  }
})(function(currentScript) {
  init.currentScript = currentScript;

  if (typeof dartMainRunner === "function") {
    dartMainRunner(F.main, []);
  } else {
    F.main([]);
  }
});
// END invoke [main].
function init() {
  Isolate.$isolateProperties = {};
  function generateAccessor(fieldDescriptor, accessors, cls) {
    var fieldInformation = fieldDescriptor.split("-");
    var field = fieldInformation[0];
    var len = field.length;
    var code = field.charCodeAt(len - 1);
    var reflectable;
    if (fieldInformation.length > 1)
      reflectable = true;
    else
      reflectable = false;
    code = code >= 60 && code <= 64 ? code - 59 : code >= 123 && code <= 126 ? code - 117 : code >= 37 && code <= 43 ? code - 27 : 0;
    if (code) {
      var getterCode = code & 3;
      var setterCode = code >> 2;
      var accessorName = field = field.substring(0, len - 1);
      var divider = field.indexOf(":");
      if (divider > 0) {
        accessorName = field.substring(0, divider);
        field = field.substring(divider + 1);
      }
      if (getterCode) {
        var args = getterCode & 2 ? "receiver" : "";
        var receiver = getterCode & 1 ? "this" : "receiver";
        var body = "return " + receiver + "." + field;
        var property = cls + ".prototype.get$" + accessorName + "=";
        var fn = "function(" + args + "){" + body + "}";
        if (reflectable)
          accessors.push(property + "$reflectable(" + fn + ");\n");
        else
          accessors.push(property + fn + ";\n");
      }
      if (setterCode) {
        var args = setterCode & 2 ? "receiver, value" : "value";
        var receiver = setterCode & 1 ? "this" : "receiver";
        var body = receiver + "." + field + " = value";
        var property = cls + ".prototype.set$" + accessorName + "=";
        var fn = "function(" + args + "){" + body + "}";
        if (reflectable)
          accessors.push(property + "$reflectable(" + fn + ");\n");
        else
          accessors.push(property + fn + ";\n");
      }
    }
    return field;
  }
  Isolate.$isolateProperties.$generateAccessor = generateAccessor;
  function defineClass(name, cls, fields) {
    var accessors = [];
    var str = "function " + cls + "(";
    var body = "";
    for (var i = 0; i < fields.length; i++) {
      if (i != 0)
        str += ", ";
      var field = generateAccessor(fields[i], accessors, cls);
      var parameter = "parameter_" + field;
      str += parameter;
      body += "this." + field + " = " + parameter + ";\n";
    }
    str += ") {\n" + body + "}\n";
    str += cls + ".builtin$cls=\"" + name + "\";\n";
    str += "$desc=$collectedClasses." + cls + ";\n";
    str += "if($desc instanceof Array) $desc = $desc[1];\n";
    str += cls + ".prototype = $desc;\n";
    if (typeof defineClass.name != "string") {
      str += cls + ".name=\"" + cls + "\";\n";
    }
    str += accessors.join("");
    return str;
  }
  var inheritFrom = function() {
    function tmp() {
    }
    var hasOwnProperty = Object.prototype.hasOwnProperty;
    return function(constructor, superConstructor) {
      tmp.prototype = superConstructor.prototype;
      var object = new tmp();
      var properties = constructor.prototype;
      for (var member in properties)
        if (hasOwnProperty.call(properties, member))
          object[member] = properties[member];
      object.constructor = constructor;
      constructor.prototype = object;
      return object;
    };
  }();
  Isolate.$finishClasses = function(collectedClasses, isolateProperties, existingIsolateProperties) {
    var pendingClasses = {};
    if (!init.allClasses)
      init.allClasses = {};
    var allClasses = init.allClasses;
    var hasOwnProperty = Object.prototype.hasOwnProperty;
    if (typeof dart_precompiled == "function") {
      var constructors = dart_precompiled(collectedClasses);
    } else {
      var combinedConstructorFunction = "function $reflectable(fn){fn.$reflectable=1;return fn};\n" + "var $desc;\n";
      var constructorsList = [];
    }
    for (var cls in collectedClasses) {
      if (hasOwnProperty.call(collectedClasses, cls)) {
        var desc = collectedClasses[cls];
        if (desc instanceof Array)
          desc = desc[1];
        var classData = desc["^"], supr, name = cls, fields = classData;
        if (typeof classData == "string") {
          var split = classData.split("/");
          if (split.length == 2) {
            name = split[0];
            fields = split[1];
          }
        }
        var s = fields.split(";");
        fields = s[1] == "" ? [] : s[1].split(",");
        supr = s[0];
        split = supr.split(":");
        if (split.length == 2) {
          supr = split[0];
          var functionSignature = split[1];
          if (functionSignature)
            desc.$signature = function(s) {
              return function() {
                return init.metadata[s];
              };
            }(functionSignature);
        }
        if (typeof dart_precompiled != "function") {
          combinedConstructorFunction += defineClass(name, cls, fields);
          constructorsList.push(cls);
        }
        if (supr)
          pendingClasses[cls] = supr;
      }
    }
    if (typeof dart_precompiled != "function") {
      combinedConstructorFunction += "return [\n  " + constructorsList.join(",\n  ") + "\n]";
      var constructors = new Function("$collectedClasses", combinedConstructorFunction)(collectedClasses);
      combinedConstructorFunction = null;
    }
    for (var i = 0; i < constructors.length; i++) {
      var constructor = constructors[i];
      var cls = constructor.name;
      var desc = collectedClasses[cls];
      var globalObject = isolateProperties;
      if (desc instanceof Array) {
        globalObject = desc[0] || isolateProperties;
        desc = desc[1];
      }
      allClasses[cls] = constructor;
      globalObject[cls] = constructor;
    }
    constructors = null;
    var finishedClasses = {};
    init.interceptorsByTag = Object.create(null);
    init.leafTags = {};
    function finishClass(cls) {
      var hasOwnProperty = Object.prototype.hasOwnProperty;
      if (hasOwnProperty.call(finishedClasses, cls))
        return;
      finishedClasses[cls] = true;
      var superclass = pendingClasses[cls];
      if (!superclass || typeof superclass != "string")
        return;
      finishClass(superclass);
      var constructor = allClasses[cls];
      var superConstructor = allClasses[superclass];
      if (!superConstructor)
        superConstructor = existingIsolateProperties[superclass];
      var prototype = inheritFrom(constructor, superConstructor);
    }
    for (var cls in pendingClasses)
      finishClass(cls);
  };
  Isolate.$lazy = function(prototype, staticName, fieldName, getterName, lazyValue) {
    var sentinelUndefined = {};
    var sentinelInProgress = {};
    prototype[fieldName] = sentinelUndefined;
    prototype[getterName] = function() {
      var result = $[fieldName];
      try {
        if (result === sentinelUndefined) {
          $[fieldName] = sentinelInProgress;
          try {
            result = $[fieldName] = lazyValue();
          } finally {
            if (result === sentinelUndefined) {
              if ($[fieldName] === sentinelInProgress) {
                $[fieldName] = null;
              }
            }
          }
        } else {
          if (result === sentinelInProgress)
            H.throwCyclicInit(staticName);
        }
        return result;
      } finally {
        $[getterName] = function() {
          return this[fieldName];
        };
      }
    };
  };
  Isolate.$finishIsolateConstructor = function(oldIsolate) {
    var isolateProperties = oldIsolate.$isolateProperties;
    function Isolate() {
      var hasOwnProperty = Object.prototype.hasOwnProperty;
      for (var staticName in isolateProperties)
        if (hasOwnProperty.call(isolateProperties, staticName))
          this[staticName] = isolateProperties[staticName];
      function ForceEfficientMap() {
      }
      ForceEfficientMap.prototype = this;
      new ForceEfficientMap();
    }
    Isolate.prototype = oldIsolate.prototype;
    Isolate.prototype.constructor = Isolate;
    Isolate.$isolateProperties = isolateProperties;
    Isolate.$finishClasses = oldIsolate.$finishClasses;
    return Isolate;
  };
}
})()

//# sourceMappingURL=out.js.map
//@ sourceMappingURL=out.js.map

And that's it, I've thrown away Dart because I didn't knew what to do with generated JS file, also I was frightened of potentially high amount of time required for keeping the resulting library interface clean and similar to one I'm using with JavaScript.

The question(s)

1. How do I expose class definitions created in Dart and later use them in JavaScript?
2. Do you think it's worth it going into Dart when nearly all potential library users will be using JS version instead? (Using dart is already not good for me due to difference in community sizes, this means that less people will find it easy to contribute to my library)
3. In your opinion, what should I do?

Answer 1

Even though Dart supports this use case, if you target JavaScript developers I would stick with JavaScript.

@AlexandreArdhuin shows in his answer to Expose Dart functions to javascript how you can make a Dart function available to JavaScript.
Under the dart-js-interop are many examples how to do function calls and pass data between Dart and JavaScript.