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dojo/_base/declare

Author:Eugene Lazutkin
since:0.9

dojo/_base/declare contains functions to define Dojo classes, which support standard Object Oriented concepts within Dojo.

JavaScript uses prototype-based inheritance, not class-based inheritance (which is used by most programming languages). Dojo provides the ability to simulate class-based inheritance using dojo.declare.

dojo/declare()

Defining a Class

In my/person.js:

define(["dojo/_base/declare"], function(declare){
  return declare(null, {
    constructor: function(name, age, residence){
      this.name = name;
      this.age = age;
      this.residence = residence;
    }
  });
});

Using the class:

require(["my/Person"], function(Person){
  var folk = new Person("phiggins", 42, "Tennessee");
});

Using mixin() in a Class Constructor

Define a class, “my/Person”, using an object for mixing in arguments instead of an argument list.

In my/person.js:

// in "my/Person.js"
define(["dojo/_base/declare", "dojo/_base/lang"], function(declare, lang){
  return declare(null, {
    name: "Anonymous",
    age: null,
    residence: "Universe A",

    constructor: function(/*Object*/ kwArgs){
      lang.mixin(this, kwArgs);
    },

    moveTo: function(/*String*/ residence){
      this.residence = residence;
    }
  });
});

Using the class:

// using the class elsewhere...
require(["my/Person"], function(Person){
  var anon  = new Person(),
      alice = new Person({ name: "Alice", age: 42, residence: "Universe 1" });

  console.log(anon.name, alice.name); // "Anonymous", "Alice"
  console.log(anon.residence, alice.residence); // "Universe A", "Universe 1"
  alice.moveTo("Universe 420");
  console.log(alice.residence); // "Universe 420"
});

Arrays and Objects as Member Variables

If your class contains arrays or other objects, they should be declared in the constructor() so that each instance gets its own copy. Simple types (strings, numbers, booleans, null) are fine to declare in the class directly because simple types are assigned by value, whereas objects are assigned by reference (in JavaScript, arrays are also considered objects).

Instance Objects

Define a class, my/Demo.js, with instance objects:

define(["dojo/_base/declare", "my/Foo"], function(declare, Foo){
  return declare(null, {
    arr: [ 1, 2, 3, 4 ], // object. shared by all instances!
    num: 5,              // non-object. not shared.
    str: "string",       // non-object. not shared.
    obj: new Foo(),      // object. shared by all instances!

    constructor: function(){
      this.arr = [ 1, 2, 3, 4 ]; // per-instance object.
      this.obj = new Foo();      // per-instance object.
    }
  });
});

Static objects

Define a class, my/Demo.js, with “static” properties:

define(['dojo/_base/declare'], function(declare){
  var Demo = declare(null, {
    constructor: function(){
      console.debug("this is Demo object #" + Demo.counter++);
    }
  });

  Demo.counter = 0;

  return Demo;
});

Inheritance

The superclass argument to the dojo/declare() function is used for extending classes by providing a parent class or classes to inherit from.

Single Inheritance

Define a my/Employee class that extends the my/Person class from above. In my/Employee.js:

define(["dojo/_base/declare", "my/Person"], function(declare, Person){
  return declare(Person, {
    constructor: function(name, age, residence, salary){
      // The "constructor" method is special: the parent class (Person)
      // constructor is called automatically before this one.

      this.salary = salary;
    },

    askForRaise: function(){
      return this.salary * 0.02;
    }
  });
});

In my/Boss.js:

define(["dojo/_base/declare", 'my/Employee'], function(declare, Employee){
  return declare(Employee, {
    askForRaise: function(){
      return this.salary * 0.25;
    }
  });
});

To use the classes:

require(["my/Employee", "my/Boss"], function(Employee, Boss){
  var kathryn = new Boss("Kathryn", 26, "Minnesota", 9000),
      matt    = new Employee("Matt", 33, "California", 1000);

  console.log(kathryn.askForRaise(), matt.askForRaise()); // 2250, 20
});

Calling Superclass Methods

Superclass constructors are always called automatically, and always before the subclass constructor. This convention reduces boilerplate in 90% of cases. If it doesn’t fit your needs see Manual Constructor Chaining below. For all other methods, use this.inherited(arguments) to call the superclass method of the same name.

In my/Boss.js:

define(["dojo/_base/declare", "my/Employee"], function(declare, Employee){
  return declare(Employee, {
    // override the askForRaise function from the Employee class
    askForRaise: function(){
      return this.inherited(arguments) * 20; // boss multiplier!
    }
  });
});

To use the class:

require(['my/Employee', 'my/Boss'], function(Employee, Boss){
  var kathryn = new Boss("Kathryn", 26, "Minnesota", 9000),
      matt    = new Employee("Matt", 33, "California", 1000);

  console.log(kathryn.askForRaise(), matt.askForRaise()); // 3600, 20
});

Note that the first argument to this.inherited() is always literally arguments, a special JavaScript array- like pseudo-variable which holds all the arguments (like argv in C). If you want to override the arguments passed to the superclass, pass them in an array as a second argument:

this.inherited(arguments, [ customArg1, customArg2 ]);

See inherited() for more details.

Multiple Inheritance

Just as Dojo adds class-based inheritance to JavaScript, so it adds support for multiple inheritance using C3 superclass linearization. This algorithm is what Python and some other languages use for its implementation of multiple inheritance. You can learn more details in the Python 2.3 Method Resolution Order documentation.

In static languages like Java, you must use typecasts to make an object “act like” its mixed-in class (in Java, this is through interfaces). In Dojo, you can use the mixed-in properties directly.

Define a my/Blizzard class using the base class VanillaSoftServe and mixins OreoMixin and CookieDoughMixin. In my/Blizzard.js:

define(["dojo/_base/declare"], function(declare){
  var VanillaSoftServe = declare(null, {
        constructor: function(){
          console.debug ("adding soft serve");
        }
      });

  var OreoMixin = declare(null, {
        constructor: function(){
          console.debug("mixing in oreos");
        },
        kind: "plain"
      });

  var CookieDoughMixin = declare(null, {
        constructor: function(){
          console.debug("mixing in cookie dough");
        },
        chunkSize: "medium"
      });
  };

  return declare([VanillaSoftServe, OreoMixin, CookieDoughMixin], {
    constructor: function(){
      console.debug("A blizzard with " +
        this.kind + " oreos and " +
        this.chunkSize + "-sized chunks of cookie dough."
      );
    }
  });
});

To use the class:

require(["my/Blizzard"], function(Blizzard){
  // This will print to console:
  // "adding soft serve",
  // "mixing in oreos",
  // "mixing in cookie dough",
  // "A blizzard with plain oreos and medium-sized chunks of cookie dough."
  var yummyTreat = new Blizzard();
});

Only the first class passed for multiple inheritance is a true superclass. The rest are mixins, and are mixed into the child class to produce the inheritance chain we need. On a practical level, this means that the instanceof operator cannot be used for mixins, only for base classes. Instead, use the isInstanceOf() function.

Signature

The signature of dojo/declare() is:

NameTypeDescription
classNameString

The optional name of the constructor (loosely, a "class") stored in the "declaredClass" property in the created prototype. It will be used as a global name for a created constructor.

superclassFunction|Function[]

May be null, a Function, or an Array of Functions. This argument specifies a list of bases (the left-most one is the most deepest base).

propsObject

An object whose properties are copied to the created prototype. Add an instance-initialization function by making it a property named "constructor".

safeMixin()

safeMixin() is a function defined in dojo/declare. It has the same functionality as dojo/_base/lang::mixin(), but additionally it annotates all copied methods compatibly with dojo/declare. This decoration can affect how this.inherited() works in mixed-in methods.

Usage

The function is usually used with classes and instances produced by dojo/declare. It takes two parameters (both objects): target and source of properties. Just like dojo/_base/lang::mixin() it returns target.

require(["dojo/_base/declare", "dojo/_base/lang"], function(declare, lang){
  var A = declare(null, {
    m1: function(){ /*...*/ },
    m2: function(){ /*...*/ },
    m3: function(){ /*...*/ },
    m4: function(){ /*...*/ },
    m5: function(){ /*...*/ }
  });

  var B = declare(A, {
    m1: function(){
      // we can do that because m1 is annotated by dojo.declare()
      return this.inherited(arguments); // calls A.m1
    }
  });

  B.extend({
    m2: function(){
      // we can do that because m2 is annotated by class.extend()
      return this.inherited(arguments); // calls A.m2
    }
  });

  lang.extend(B, {
    m3: function(){
      // we have to specify the name because
      // this method is not annotated properly
      return this.inherited("m3", arguments); // calls A.m3
  });

  var x = new B();

  declare.safeMixin(x, {
    m4: function(){
      // we can do that because m4 is annotated by dojo.safeMixin()
      return this.inherited(arguments); // calls A.m4
    }
  });

  lang.mixin(x, {
    m5: function(){
      // we have to specify the name because
      // this method is not annotated properly
      return this.inherited("m5", arguments); // calls A.m5
  });
});

Notes

Common Conventions

There are some common conventions used in Dojo Toolkit which are not strictly mandated by the class system, but coupled with AMD make the creation of classes more manageable and portable:

  • The class name (first argument of declare()) is omitted in new development. This ensures that the global namespace does not become polluted with classes and reduces the chances of name collisions. Classes in Dojo Toolkit introduced prior to Dojo 1.7 will include the class name for backwards compatability reasons.
  • The AMD Module ID (MID) becomes the commonly referred to class name. This name is implied from the path to the file. For example package/Class.js would make the MID package/Class.
  • Classes that can be instantiated are named in a UpperCamelCased format.
  • “Private” classes and mixins, that are not intended to be instantiated directly by an end-developer are prefaced with a underscore (_). For example _MyPrivateClass.
  • Classes instantied have 1 to 1 relationship with modules and should constitute the return value of the define() callback.
  • Classes that are private and do not need to be instantiated outside of the module should be declared as variables within the module. For example in a file name package/MyCollection.js:
define(["dojo/_base/declare"], function(declare){
  var _MyPrivateItem = declare(null, {
    someProperty: null
  });

  return declare(null, {
    item: null,

    constructor: function(){
      this.item = new _MyPRivateItem();
    }
  });
});

Inheritance Info

dojo/declare() uses C3 superclass linearization to convert multiple inheritance to a linear list of superclasses. While it solves most thorny problems of inheritance, some configurations are impossible:

require(["dojo/_base/declare"], function(declare){
  var A = declare(null);
  var B = declare(null);
  var C = declare([A, B]);
  var D = declare([B, A]);
  var E = declare([C, D]);
});

As you can see D requires that B should go before A, and C requires that A go before B. It makes an inheritance chain for E impossible because these contradictory requirements cannot be satisfied. Obviously any other circular dependencies cannot be satisfied either. But any DAG inheritance will be linearized correctly including the famous Diamond problem.

In same rare cases it is possible to build a linear chain, which cannot reuse the base class:

require(["dojo/_base/declare"], function(declare){
  // the first batch
  var A = declare(null);
  var B = declare(A);
  var C = declare(B);

  // the second batch
  var D = declare(null);
  var E = declare([D, B]);

  // the quirky case
  var F = declare([C, E]);
});

Let’s look at C and E inheritance chains:

C -> B -> A
E -> B -> D

As you can see in one case B follows after A and in the other case it follows D. How does F look like?

F -> C -> B -> D -> A

As you can see all dependency rules are satisfied, yet the chain's tail doesn't match C as we are accustomed to see. Obviously instanceof would be useless in this case, but isInstanceOf() will work just fine. So when in doubt use isInstanceOf().

Chaining

By default only constructors are chained automatically. In some cases user may want to chain other methods too (e.g. life-cycle methods), which govern how instances are created, modified, and destroy, or methods called for various events. Good example is destroy() method, which destroys external objects and references and can be used by all super classes of an object.

While this.inherited() takes care of all scenarios, chaining has following benefits:

  • It is much faster than using this.inherited(). On some browsers the difference can be more than an order of magnitude for simple methods.
  • It is automatic. User cannot forget to call a superclass method.
  • Less code to write, less code to worry about.

Chained methods should not return values, all returned values are going to be ignored. They all be called with the same arguments. A good practice is to avoid modifications to the arguments. It will ensure that your classes play nice with others when used as superclasses.

There are two ways to chain methods: after and before (AOP terminology is used). after means that a method is called after its superclass' method. before means that a method is called before calling its superclass method. All chains are described in a special property named -chains-:

require(["dojo/_base/declare", "dojo/dom-construct", "dojo/_base/window"],
function(declare, domConst, win){
  var A = declare(null, {
    "-chains-": {
      init:    "after",
      destroy: "before"
    },

    init: function(token){
      this.initialized = true;
      this.token = token;
      this.node = domConst.create("div", null, win.body());
      console.log("A.init");
    },

    destroy: function(){
      domConst.destroy(this.node);
      this.node = null;
      console.log("A.destroy");
    }
  });

  var B = declare(A, {
    init: function(token){
      console.log("B.init");
      // more code
    },

    destroy: function(){
      console.log("B.destroy");
      // more code
    }
  });

  var x = new B();
  x.init(42);
  x.destroy();
});

// prints:
// A.init
// B.init
// B.destroy
// A.destroy

Chain declarations are inherited. Chaining for individual methods can be overridden in child classes, but not advised.

There is a special case: chain declaration for constructor. This method supports two chaining directives: after, and manual. See more details in Constructors.

Constructors

Constructor invocations are governed by Chaining.

Default Constructor Chaining

By default all constructors are chained using after algorithm (using AOP terminology). It means that after the linearization for any given class its constructor is going to be called after its superclass constructors:

require(["dojo/_base/declare"], function(declare){
  var A = declare(null, {
    constructor: function(){ console.log("A"); }
  });

  var B = declare(A, {
    constructor: function(){ console.log("B"); }
  });

  var C = declare(B, {
    constructor: function(){ console.log("C"); }
  });

  new C();
});

// prints:
// A
// B
// C

The exact algorithm of an instance initialization for chained constructors:

Notes:

  • A good practice for constructors is to avoid modifications of its arguments. It ensures that other classes can access original values, and allows to play nice when the class is used as a building block for other classes.
  • If you do need to modify arguments of superclass constructors consider Manual Constructor Chaining as a better alternative to preamble().
  • If a class doesn't use preamble() it switches the initialization to the fast path making an instantiation substantially faster.
  • For historical reasons preamble() is called for classes without a constructor and even for the last class in the superclass list, which doesn't have a superclass.

Manual Constructor Chaining

In some cases users may want to redefine how initialization works. In this case the chaining should be turned off so this.inherited() can be used instead.

require(["dojo/_base/declare"], function(declare){
  var A = declare(null, {
    constructor: function(){
      console.log("A");
    }
  });

  var B = declare(A, {
    "-chains-": {
      constructor: "manual"
    },
    constructor: function(){
      console.log("B");
    }
  });

  var C = declare(B, {
    constructor: function(){
      console.log("C - 1");
      this.inherited(arguments);
      console.log("C - 2");
    }
  });

  var x = new C();
});

// prints:
// C - 1
// B
// C - 2

The example above doesn't call the constructor of A at all, and runs some code before and after calling the constructor of B.

The exact algorithm of an instance initialization for manual constructors:

Notes:

  • Prefer manual constructors to deprecated preamble().
  • As soon as you switch to manual constructors all constructors in your hierarchy should be called manually. Make sure that all constructors are wired for that.
  • Chaining works faster than simulating it with this.inherited(). Know when to use it.

Constructor Methods

Every constructor created by dojo/declare() defines some convenience methods.

extend()

This constructor method adds new properties to the constructor's prototype the same way as dojo/_base/lang::extend() works. The difference is that it annotates function properties the same way dojo/declare() does. These changes will be propagated to all classes and object where this class constructor was a superclass.

The method has one argument, an object to mix in. It returns the constructor itself, which can be used for chained calls. For example:

require(["dojo/_base/declare"], function(declare){
  var A = declare(null, {
    m1: function(){
      // ...
    }
  });

  A.extend({
    m1: function(){
      // this method will replace the original method
      // ...
    },

    m2: function(){
      // ...
    }
  });

  var x = new A();
  a.m1();
  a.m2();
});

Internally this method uses safeMixin().

Note Do not forget that dojo/declare() uses mixins to build a constructor from several bases. Remember that only the first base is the true superclass, the rest is mixed in by copying properties. It means that if you extend() a constructor's prototype that was already used as a mixin and its methods became top methods in the chain of inheritance, these top methods would not be replaced because they are already copied. For example:

require(["dojo/_base/declare"], function(declare){
  var A = declare(null, {
    m1: function(){ console.log("A org"); },
    m2: function(){ console.log("A org"); }
  });

  var B = declare(null, {
    m2: function(){ this.inherited(arguments); console.log("B org"); },
    m3: function(){ this.inherited(arguments); console.log("B org"); }
  });

  var C = declare(null, {
    m3: function(){ this.inherited(arguments); console.log("C org"); },
    m4: function(){ this.inherited(arguments); console.log("C org"); }
  });

  var ABC = declare([A, B, C], {});

  // now A is the true base, B and C are mixed in

  var abc = new ABC();

  abc instanceof A; // true
  abc instanceof B; // false
  abc instanceof C; // false

  // use isInstanceOf() to check if you include
  // proper mixins

  // let's list top methods:
  // m1 comes from A (inherited)
  // m2 comes from B (copied)
  // m3 comes from C (copied)
  // m4 comes from D (copied)

  abc.m1(); // A org
  abc.m2(); // A org, B org
  abc.m3(); // B org, C org
  abc.m4(); // C org

  // let's extend() all prototypes

  A.extend({
    m1: function(){ console.log("A new"); },
    m2: function(){ console.log("A new"); }
  });

  B.extend({
    m2: function(){ this.inherited(arguments); console.log("B new"); },
    m3: function(){ this.inherited(arguments); console.log("B new"); }
  });

  C.extend({
    m3: function(){ this.inherited(arguments); console.log("C new"); },
    m4: function(){ this.inherited(arguments); console.log("C new"); }
  });

  // observe that top copied methods are not changed

  abc.m1(); // A new
  abc.m2(); // A new, B org
  abc.m3(); // B new, C org
  abc.m4(); // C org
});

You can see that copied methods were not replaced in ABC and abc.

Class Methods

Every prototype produced by dojo/declare() contains some convenience methods.

inherited()

The method is used to call a superclass method. It accepts up to three arguments:

  • Optional name of the method to call. Generally it should be specified when calling this.inherited() from an un- annotated method, otherwise it will be deduced from the method itself.
  • arguments - literally arguments pseudo-variable, which is used for introspection.
  • Optional array of arguments, which will be used to call a superclass method. If it is not specified arguments are used. If this argument is a literal constant true, then the found super method is not executed but returned as a value (see getInherited()).

It returns whatever value was returned by a superclass method that was called. If it turned out that there is no superclass method to call, inherited() doesn't do anything and returns undefined. For example:

require(["dojo/_base/lang", "dojo/_base/declare"], function(lang, declare){
  var A = declare(null, {
    m1: function(){
      // ...
    },
    m2: function(){
      // ...
    },
    m3: function(){
      // ...
    },
    m4: function(){
      // ...
    },
    m5: function(){
      // ...
    }
  });

  var B = declare(A, {
    m1: function(){
      // simple super call with the same arguments
      this.inherited(arguments);
      // super call with new arguments
      this.inherited(arguments, [1, 2, 3]);
    }
  });

  // extend B using extend()
  B.extend({
    m2: function(){
      // this method is going to be properly annotated =>
      // we can use the same form of this.inherited() as
      // normal methods:
      // simple super call with the same arguments
      this.inherited(arguments);
      // super call with new arguments
      this.inherited(arguments, ["a"]);
    }
  });

  // extend B using lang.extend()
  lang.extend(B, {
    m3: function(){
      // this method is not annotated =>
      // we should supply its name when calling
      // a superclass:
      // simple super call with the same arguments
      this.inherited("m3", arguments);
      // super call with new arguments
      this.inherited("m3", arguments, ["a"]);
    }
  });

  // let's create an instance
  var x = new B();
  x.m1();
  x.m2();
  x.m3();
  x.m4(); // A.m4() is called
  x.m5(); // A.m5() is called

  // add a method on the fly using declare.safeMixin()
  declare.safeMixin(x, {
    m4: function(){
      // this method is going to be properly annotated =>
      // we can use the same form of this.inherited() as
      // normal methods:
      // simple super call with the same arguments
      this.inherited(arguments);
      // super call with new arguments
      this.inherited(arguments, ["a"]);
    }
  });

  // add a method on the fly
  x.m5 = function(){
    // this method is not annotated =>
    // we should supply its name when calling
    // a superclass:
    // simple super call with the same arguments
    this.inherited("m5", arguments);
    // super call with new arguments
    this.inherited("m5", arguments, ["a"]);
  };

  x.m4(); // our instance-specific method is called
  x.m5(); // our instance-specific method is called
});

getInherited()

This is a companion method to inherited(). The difference is that it doesn't execute the found method, but returns it. It is up to the user to call it with proper arguments.

The method accepts up to two arguments:

  • Optional name of the method to call. If it is specified it must match the name of the caller. Generally it should be specified when calling this method from an un-annotated method (the same rule as for inherited()).
  • arguments - literally arguments pseudo-variable, which is used for introspection.

The result is a superclass method or undefined, if it was not found. You can use the result as you wish. The most useful case is to pass it to some other function, which cannot use inherited() directly for some reasons. For example:

require(["dojo/_base/declare"], function(declare){
  var A = declare(null, {
    m1: function(){
      // ...
    },
    m2: function(){
      // ...
    }
  });

  var B = declare(A, {
    logAndCall: function(name, method, args){
      console.log("Calling " + name + "...");
      method.apply(this, args);
      console.log("...done");
    },
    m1: function(){
      var supermethod = this.getInherited(arguments);
      this.logAndCall("A.m1", supermethod, [1, 2]);
    }
  });

  var x = new B();
  x.m2 = function(){
    // we need to use a name here because
    // this method was not properly annotated:
    var supermethod = this.getInherited("m2", arguments);
    this.logAndCall("A.m2", supermethod, [1, 2]);
  };
});

Internally this method is a helper, which calls inherited() with true as the last argument.

isInstanceOf()

This method checks if an instance is derived from a given class. It is modeled on instanceof operator. It is most useful when you have classes built with the multiple inheritance somewhere in your hierarchy.

The method accepts one argument: class (constructor). It returns true/false. For example:

require(["dojo/_base/declare"], function(declare){
  var A = declare(null);
  var B = declare(null);
  var C = declare(null);

  var D = declare([A, B]);

  var x = new D();

  console.log(x instanceof A);     // true
  console.log(x.isInstanceOf(A));  // true

  console.log(x instanceof B);     // false
  console.log(x.isInstanceOf(B));  // true

  console.log(x instanceof C);     // false
  console.log(x.isInstanceOf(C));  // false

  console.log(x instanceof D);     // true
  console.log(x.isInstanceOf(D));  // true
});

Using "Raw" Classes

dojo/declare() allows to use "raw" classes created by other means as a superclass. Such classes are considered to be monolithic (because their structure cannot be introspected) and they cannot use advanced features like inherited(). But their methods will be called by inherited() and all their methods can be chained (see Chaining) including constructors. For example:

require(["dojo/_base/lang", "dojo/_base/declare"], function(lang, declare){
  // plain vanilla constructor
  var A = function(){
    this.a = 42;
  };
  A.prototype.m1 = function(){
    // ...
  };

  // another plain vanilla constructor
  var B = function(){
    this.b = "abc";
  };
  lang.extend(B, {
    m2: function(){
      // ...
    }
  });

  var C = declare([A, B], {
    m1: function(){
      return this.inherited(arguments);
    },
    m2: function(){
      return this.inherited(arguments);
    }
  });

  var x = new C();
  // both A and B will be called at this point

  console.log(x.isInstanceOf(A)); // true
  console.log(x.isInstanceOf(B)); // true

  x.m1(); // A.m1 will be called via this.inherited()
  x.m2(); // B.m2 will be called via this.inherited()
});

Meta-Information

All meta-information is a subject to change and should not be used in the course of normal coding. If you use it, be ready to update your code, when it changes.

Every constructor produced with dojo/declare() carries a meta-information required for internal plumbing and for introspection. It is implemented as a property called meta on a constructor. meta has following properties:

bases
List of all superclasses produced by the C3 linearization algorithm (see Inheritance for more details). The very first item in the list is the class itself.
hidden
Copy of all own properties and methods of the class. It is the third argument (or the second argument, if class name was omitted) of dojo/declare().
chains
List of chains (see Chaining for more details) augmented by all inherited chains.
parents
List of immediate parents. It is the second argument (or the first argument, if class name was omitted) of dojo/declare().

Additionally a prototype has a special property named declaredClass, if the class was named when created by dojo/declare(). If it was an anonymous class, this property can be missing, or it can be a auto-generated name in the form of uniqName_NNN, where NNN is some unique number. This property is used internally to distinguish between different classes. It is not meant for end users, but it can be useful for debugging.

Every instance created by a dojo/declare() class has a special property called inherited, which is used to speed up inherited() calls. Please don't touch it.

Every method mixed in by dojo/declare() or safeMixin() is annotated: a special property called nom is added. It contains a name of the method in question and used by inherited() and getInherited() to deduce the name of a superclass method. See safeMixin() for more details.

safeMixin() Information

While copying properties safeMixin() (and dojo/declare()) annotates methods. All other properties are copied unmodified. On any function it adds a single property: nom, which value is a name of the function property.

require(["dojo/_base/declare"], function(declare){
  var x = {};
  declare.safeMixin(x, {
    a: 1,
    b: "two",
    c: {
       x: 42
    },
    d: function(){}
  });

  console.log(x.d.nom); // prints: d
});

This way this.inherited() and this.getInherited() know what superclass method to call. If this property is not there, you have to specify the name as the first argument in this.inherited() or this.getInherited().

JavaScript treats functions as objects (not values) and uses them by reference. It means that if you add a function to two (or more) objects, it will be annotated several times leading to wrong annotations in different contexts:

require(["dojo/_base/declare"], function(declare){
  var fun = function(){
    this.inherited(arguments);
  };

  var x = {}, y = {};

  declare.safeMixin(x, {doSomething: fun});
  console.log(fun.nom);            // doSomething
  console.log(x.doSomething.nom);  // doSomething

  declare.safeMixin(y, {anotherName: fun});
  console.log(fun.nom);            // anotherName
  console.log(y. anotherName.nom); // anotherName

  console.log(x.doSomething.nom);  // anotherName
});

As you can see we reused the same function as a method, and it was annotated twice. It will break this.inherited() and this.getInherited() in all objects but the last one.

How to prevent this problem?

  • Try to avoid these situations. In most cases it can be done easily because functions are frequently created from literals and not reused in this manner (99% of all cases).
  • Use function wrappers. Downside: it introduces an extra function call, which may affect the performance of short fast methods.
  • Use a name in calls to this.inherited() and this.getInherited(). Downside: you have to know the name, and it is not always possible.