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<td >Qt 5.12</td><td ><a href="qtqml-index.html">Qt QML</a></td><td ><a href="qtqml-qmlmodule.html">QML Types</a></td><td >var QML Basic Type</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right"><a href="qtqml-index.html">Qt 5.12.6 Reference Documentation</a></td>
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<h3><a name="toc">Contents</a></h3>
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<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>
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<h1 class="title">var QML Basic Type</h1>
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<p>a generic property type.</p>
<p>The <code>var</code> 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>
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<h2 id="change-notification-semantics">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 "The car has 4 wheels" as the change to the wheels property will not cause the reevaluation of the binding assigned to the "text" property:</p>
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<p>If the onCompleted handler instead had <code>"car = new Object({wheels: 6})"</code> then the text would be updated to say "The car has 6 wheels", since the car property itself would be changed, which causes a change notification to be emitted.</p>
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<h2 id="property-value-initialization-semantics">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 <code>var</code> property, as empty curly braces in JavaScript can denote either an expression block or an empty object declaration. If you wish to initialize a <code>var</code> property to an empty object value, you should wrap the curly braces in parentheses.</p>
<p>For example:</p>
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<p>In the previous example, the <code>first</code> property is bound to an empty expression, whose result is undefined. The <code>second</code> property is bound to an expression which contains a single, empty expression block ("{}"), which similarly has an undefined result. The <code>third</code> 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>
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<p><b>See also </b><a href="qtqml-typesystem-basictypes.html">QML Basic Types</a>.</p>
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