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<?xml version="1.0" encoding="UTF-8"?>
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<!-- basictypes.qdoc -->
  <title>QML Basic Type: var | QtQml 5.2</title>
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<div class="header" id="qtdocheader">
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        <ul>
<li>Qt 5.2</li>
<li><a href="qtqml-index.html">Qt QML</a></li>
<li><a href="qtqml-qmlmodule.html">QML Types</a></li>
<li>QML Basic Type: var</li>
<li id="buildversion">
Qt 5.2.1 Reference Documentation</li>
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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#change-notification-semantics">Change Notification Semantics</a></li>
<li class="level1"><a href="#property-value-initialization-semantics">Property Value Initialization Semantics</a></li>
</ul>
</div>
<h1 class="title">QML Basic Type: var</h1>
<span class="subtitle"></span>
<!-- $$$var-description -->
<div class="descr"> <a name="details"></a>
<p>The <tt>var</tt> type is a generic property type that can refer to any data type.</p>
<p>It is equivalent to a regular JavaScript variable. For example, var properties can store numbers, strings, objects, arrays and functions:</p>
<pre class="qml"><span class="type">Item</span> {
    property <span class="type">var</span> <span class="name">aNumber</span>: <span class="number">100</span>
    property <span class="type">var</span> <span class="name">aBool</span>: <span class="number">false</span>
    property <span class="type">var</span> <span class="name">aString</span>: <span class="string">&quot;Hello world!&quot;</span>
    property <span class="type">var</span> <span class="name">anotherString</span>: <span class="name">String</span>(<span class="string">&quot;#FF008800&quot;</span>)
    property <span class="type">var</span> <span class="name">aColor</span>: <span class="name">Qt</span>.<span class="name">rgba</span>(<span class="number">0.2</span>, <span class="number">0.3</span>, <span class="number">0.4</span>, <span class="number">0.5</span>)
    property <span class="type">var</span> <span class="name">aRect</span>: <span class="name">Qt</span>.<span class="name">rect</span>(<span class="number">10</span>, <span class="number">10</span>, <span class="number">10</span>, <span class="number">10</span>)
    property <span class="type">var</span> <span class="name">aPoint</span>: <span class="name">Qt</span>.<span class="name">point</span>(<span class="number">10</span>, <span class="number">10</span>)
    property <span class="type">var</span> <span class="name">aSize</span>: <span class="name">Qt</span>.<span class="name">size</span>(<span class="number">10</span>, <span class="number">10</span>)
    property <span class="type">var</span> <span class="name">aVector3d</span>: <span class="name">Qt</span>.<span class="name">vector3d</span>(<span class="number">100</span>, <span class="number">100</span>, <span class="number">100</span>)
    property <span class="type">var</span> <span class="name">anArray</span>: [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="string">&quot;four&quot;</span>, <span class="string">&quot;five&quot;</span>, (<span class="keyword">function</span>() { <span class="keyword">return</span> <span class="string">&quot;six&quot;</span>; })]
    property <span class="type">var</span> <span class="name">anObject</span>: { &quot;foo&quot;: <span class="number">10</span>, &quot;bar&quot;: <span class="number">20</span> }
    property <span class="type">var</span> <span class="name">aFunction</span>: (<span class="keyword">function</span>() { <span class="keyword">return</span> <span class="string">&quot;one&quot;</span>; })
}</pre>
<a name="change-notification-semantics"></a>
<h2>Change Notification Semantics</h2>
<p>It is important to note that changes in regular properties of JavaScript objects assigned to a var property will <b>not</b> trigger updates of bindings that access them. The example below will display &quot;The car has 4 wheels&quot; as the change to the wheels property will not cause the reevaluation of the binding assigned to the &quot;text&quot; property:</p>
<pre class="qml"><span class="type">Item</span> {
    property <span class="type">var</span> <span class="name">car</span>: new <span class="name">Object</span>({wheels: <span class="number">4</span>})

    <span class="type">Text</span> {
        <span class="name">text</span>: <span class="string">&quot;The car has &quot;</span> <span class="operator">+</span> <span class="name">car</span>.<span class="name">wheels</span> <span class="operator">+</span> <span class="string">&quot; wheels&quot;</span>;
    }

    <span class="name">Component</span>.onCompleted: {
        <span class="name">car</span>.<span class="name">wheels</span> <span class="operator">=</span> <span class="number">6</span>;
    }
}</pre>
<p>If the onCompleted handler instead had <tt>&quot;car = new Object({wheels: 6})&quot;</tt> then the text would be updated to say &quot;The car has 6 wheels&quot;, since the car property itself would be changed, which causes a change notification to be emitted.</p>
<a name="property-value-initialization-semantics"></a>
<h2>Property Value Initialization Semantics</h2>
<p>The QML syntax defines that curly braces on the right-hand-side of a property value initialization assignment denote a binding assignment. This can be confusing when initializing a <tt>var</tt> property, as empty curly braces in JavaScript can denote either an expression block or an empty object declaration. If you wish to initialize a <tt>var</tt> property to an empty object value, you should wrap the curly braces in parentheses.</p>
<p>For example:</p>
<pre class="qml"><span class="type">Item</span> {
    property <span class="type">var</span> <span class="name">first</span>:  {}   <span class="comment">// nothing = undefined</span>
    property <span class="type">var</span> <span class="name">second</span>: {{}} <span class="comment">// empty expression block = undefined</span>
    property <span class="type">var</span> <span class="name">third</span>:  ({}) <span class="comment">// empty object</span>
}</pre>
<p>In the previous example, the <tt>first</tt> property is bound to an empty expression, whose result is undefined. The <tt>second</tt> property is bound to an expression which contains a single, empty expression block (&quot;{}&quot;), which similarly has an undefined result. The <tt>third</tt> property is bound to an expression which is evaluated as an empty object declaration, and thus the property will be initialized with that empty object value.</p>
<p>Similarly, a colon in JavaScript can be either an object property value assignment, or a code label. Thus, initializing a var property with an object declaration can also require parentheses:</p>
<pre class="qml"><span class="type">Item</span> {
    property <span class="type">var</span> <span class="name">first</span>: { <span class="name">example</span>: <span class="string">'true'</span> }    <span class="comment">// example is interpreted as a label</span>
    property <span class="type">var</span> <span class="name">second</span>: ({ example: <span class="string">'true'</span> }) <span class="comment">// example is interpreted as a property</span>
    property <span class="type">var</span> <span class="name">third</span>: { 'example': <span class="string">'true'</span> }  <span class="comment">// example is interpreted as a property</span>
    <span class="name">Component</span>.onCompleted: {
        <span class="name">console</span>.<span class="name">log</span>(<span class="name">first</span>.<span class="name">example</span>) <span class="comment">// prints 'undefined', as &quot;first&quot; was assigned a string</span>
        <span class="name">console</span>.<span class="name">log</span>(<span class="name">second</span>.<span class="name">example</span>) <span class="comment">// prints 'true'</span>
        <span class="name">console</span>.<span class="name">log</span>(<span class="name">third</span>.<span class="name">example</span>) <span class="comment">// prints 'true'</span>
    }
}</pre>
</div>
<p><b>See also </b><a href="qtqml-typesystem-basictypes.html">QML Basic Types</a>.</p>
<!-- @@@var -->
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