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It is typically used to:</p> <ul> <li>Add or update other properties as side effects</li> <li>Coerce the set value into an appropriate action</li> </ul> <p>The behavior of the setter depends on the number of arguments specified. This means that a setter like:</p> <pre><code class="language-js">prop: { set: function(){} } </code></pre> <p>behaves differently than:</p> <pre><code class="language-js">prop: { set: function(newVal){} } </code></pre> <div class="parameters"> <h3 class="parameters-title">Parameters</h3> <ol> <li><b>newVal</b> <code>{*}</code>: <p>The <a href="can-define.types.type.html" title="Converts a value set on an instance into an appropriate value.">type function</a> coerced value the user intends to set on the instance.</p> </li> <li><b>resolve</b> <code>{function(newValue)}</code>: <p>A callback that can set the value of the property asynchronously.</p> </li> </ol> </div> <div class="returns"> <h3 class="returns-title">Returns</h3> <p> <code>{*|undefined}</code>: <p>If a non-undefined value is returned, that value is set as the attribute value.</p> <p>If an <code>undefined</code> value is returned, the behavior depends on the number of arguments the setter declares:</p> <ul> <li>If the setter <em>does not</em> specify the <code>newValue</code> argument, the property value is set to the type converted value.</li> <li>If the setter specifies the <code>newValue</code> argument only, the attribute value will be set to <code>undefined</code>.</li> <li>If the setter specifies both <code>newValue</code> and <code>resolve</code>, the value of the property will not be updated until <code>resolve</code> is called.</li> </ul> </p> </div> </div> <section class="body"> <h2>Use</h2> <p>A property's <code>set</code> function can be used to customize the behavior of when an attribute value is set. Lets see some common cases:</p> <h4>Side effects</h4> <p>The following makes setting a <code>page</code> property update the <code>offset</code>:</p> <pre><code class="language-js">page: { set: function(newVal){ this.offset = (parseInt(newVal) - 1) * this.limit; } } </code></pre> <p>The following makes changing <code>makeId</code> un-define the <code>modelId</code> property:</p> <pre><code>makeId: { set: function(newValue){ // Check if we are changing. if(newValue !== this.makeId) { this.modelId = undefined; } // Must return value to set as we have a `newValue` argument. return newValue; } } </code></pre> <h4>Asynchronous Setter</h4> <p>The following shows an async setter:</p> <pre><code class="language-js">prop: { set: function( newVal, setVal){ $.get("/something", {}, setVal ); } } </code></pre> <h2>Behavior depends on the number of arguments.</h2> <p>When a setter returns <code>undefined</code>, its behavior changes depending on the number of arguments.</p> <p>With 0 arguments, the original set value is set on the attribute.</p> <pre><code class="language-js">MyMap = DefineMap.extend({ prop: {set: function(){}} }) var map = new MyMap({prop : "foo"}); map.prop //-> "foo" </code></pre> <p>With 1 argument, an <code>undefined</code> return value will set the property to <code>undefined</code>.</p> <pre><code class="language-js">MyMap = DefineMap.extend({ prop: {set: function(newVal){}} }) var map = new MyMap({prop : "foo"}); map.prop //-> undefined </code></pre> <p>With 2 arguments, <code>undefined</code> leaves the property in place. It is expected that <code>resolve</code> will be called:</p> <pre><code class="language-js">MyMap = DefineMap.extend({ prop: { set: function(newVal, resolve){ setVal(newVal+"d"); } } }); var map = new MyMap({prop : "foo"}); map.prop //-> "food"; </code></pre> <h2>Side effects</h2> <p>A set function provides a useful hook for performing side effect logic as a certain property is being changed.</p> <p>For example, in the example below, Paginator DefineMap includes a <code>page</code> property, which derives its value entirely from other properties (limit and offset). If something tries to set the <code>page</code> directly, the set method will set the value of <code>offset</code>:</p> <pre><code class="language-js">var Paginate = DefineMap.extend({ limit: 'number', offset: 'number', page: { set: function (newVal) { this.offset = (parseInt(newVal) - 1) * this.limit; }, get: function () { return Math.floor(this.offset / this.limit) + 1; } } }); var p = new Paginate({limit: 10, offset: 20}); </code></pre> <h2>Merging</h2> <p>By default, if a value returned from a setter is an object the effect will be to replace the property with the new object completely.</p> <pre><code class="language-js">var Contact = DefineMap.extend({ info: { set: function(newVal){ return newVal; } } }) var alice = new Contact({ info: {name: 'Alice Liddell', email: 'alice@liddell.com'} }); var info = alice.info; alice.info = {name: 'Allison Wonderland', phone: '888-888-8888'}; info === alice.info // -> false </code></pre> <p>In contrast, you can merge properties with:</p> <pre><code class="language-js">Contact = DefineMap.extend({ info: { set: function(newVal){ if(this.info) { return this.info.set(newVal); } else { return newVal; } } } }); var alice = new Contact({ info: {name: 'Alice Liddell', email: 'alice@liddell.com'} }); var info = alice.info; alice.info = {name: 'Allison Wonderland', phone: '888-888-8888'}; info === alice.info // -> true </code></pre> <h2>Batched Changes</h2> <p>By default, calls to <code>set</code> methods are wrapped in a call to <a href="can-event/batch/batch.start.html" title="Begin an event batch.">canBatch.start</a> and <a href="can-event/batch/batch.stop.html" title="End an event batch.">canBatch.stop</a>, so if a set method has side effects that set more than one property, all these sets are wrapped in a single batch for better performance.</p> </section> <script type="text/javascript"> var docObject = {"src":{"path":"node_modules/can-define/docs/types.set.md"},"description":"\nSpecify what happens when a property value is set.\n","title":"set","name":"can-define.types.set","type":"function","parent":"can-define.behaviors","signatures":[{"code":"set( [newVal,] [resolve] )","description":"\n\nA set function defines the behavior of what happens when a value is set on an\ninstance. 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