Boolean

If true, in development, instances of this object will be sealed. In strict mode errors will be thrown when undefined properties are set. This is the default behavior of extended DefineMaps:

"use strict";
var Person = DefineMap.extend({});
var me = new Person();
me.age = 33 //-> throws "TypeError: Can't add property age, object is not extensible"

If false, the object will not be sealed. This is the default behavior of unextended DefineMaps. Use get and set to get and set values:

var person = new DefineMap();
person.set("first","Justin");
person.set("last","Meyer");

person.get("first") //-> "Justin"
person.get("last") //-> "Meyer"

It is also possible to extend a sealed object and unseal it:

"use strict";
var Person = DefineMap.extend({});
var Programmer = Person.extend({seal: false}, {})
var me = new Programmer();
me.age = 33; // no error thrown

Set seal to false on objects that have an indeterminate number of properties:

var Style = DefineMap.extend({
  seal: false
},{
  cssText: {
    get: function(){
      return _.map(this.get(), function(val, prop){
        return prop+": "+val;
      }).join(";")
    }
  }
});
var style = new Style();
style.set("color","green");
style.set("font","awesome");
style.cssText //-> "color:green; font: awesome;"