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In <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode">strict mode</a> errors will be thrown when undefined properties are set. This is the default behavior of <a href="map.extend.html" title="Define a custom map type.">extended DefineMaps</a>:</p> <pre><code class="language-js">"use strict"; var Person = DefineMap.extend({}); var me = new Person(); me.age = 33 //-> throws "TypeError: Can't add property age, object is not extensible" </code></pre> <p>If <code>false</code>, the object will not be sealed. This is the default behavior of unextended <a href="map.html" title="Create observable objects.">DefineMaps</a>. Use <a href="map.prototype.get.html" title="Get a value or all values from a DefineMap.">get</a> and <a href="map.prototype.set.html" title="Sets multiple properties on a map instance or a property that wasn't predefined.">set</a> to get and set values:</p> <pre><code class="language-js">var person = new DefineMap(); person.set("first","Justin"); person.set("last","Meyer"); person.get("first") //-> "Justin" person.get("last") //-> "Meyer" </code></pre> <p>Set <code>seal</code> to <code>false</code> on objects that have an indeterminate number of properties:</p> <pre><code class="language-js">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;" </code></pre> </div> <section class="body"> </section> <script type="text/javascript"> var docObject = {"src":{"path":"node_modules/can-define/map/docs/static.seal.md"},"description":"Defines if instances of the map should be [sealed](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/seal) in development. \n","types":[{"type":"Boolean","description":"If `true`, in development, instances of this object will be [sealed](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/seal). In [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode) errors will be thrown when undefined properties are set. This is the default\nbehavior of [can-define/map/map.extend extended DefineMaps]:\n\n```js\n\"use strict\";\nvar Person = DefineMap.extend({});\nvar me = new Person();\nme.age = 33 //-> throws \"TypeError: Can't add property age, object is not extensible\"\n```\n\nIf `false`, the object will not be sealed. This is the default behavior of\nunextended [can-define/map/map DefineMaps]. Use [can-define/map/map.prototype.get] and [can-define/map/map.prototype.set] to get and set values:\n\n```js\nvar person = new DefineMap();\nperson.set(\"first\",\"Justin\");\nperson.set(\"last\",\"Meyer\");\n\nperson.get(\"first\") //-> \"Justin\"\nperson.get(\"last\") //-> \"Meyer\"\n```\n\nSet `seal` to `false` on objects that have an indeterminate number of properties:\n\n```js\nvar Style = DefineMap.extend({\n seal: false\n},{\n cssText: {\n get: function(){\n return _.map(this.get(), function(val, prop){\n return prop+\": \"+val;\n }).join(\";\")\n }\n }\n});\nvar style = new Style();\nstyle.set(\"color\",\"green\");\nstyle.set(\"font\",\"awesome\");\nstyle.cssText //-> \"color:green; font: awesome;\"\n```\n"}],"title":"seal","name":"can-define/map/map.seal","type":"property","parent":"can-define/map/map.static","pathToRoot":"../../.."}; </script> </article> <footer><p>CanJS is part of <a href="http://donejs.com" target="_blank">DoneJS</a>. 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