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";
const Person = DefineMap.extend( {} );
const 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:

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

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

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