object.html

javascript 1.5规范

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<A NAME="Description"> </A>
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<zrefheader>Description</zrefheader>
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<A NAME="1193527"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>The JavaScript debugger has functionality similar to that provided by this method, as well as other debugging options. For information on the debugger, see <I><A HREF="http://developer.netscape.com/docs/manuals/jsdebug/index.htm">Getting Started with Netscape JavaScript Debugger</A></I>.</zparagraph>
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<A NAME="1200644"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>By default, this method is inherited by every object descended from <FONT FACE="courier, couriernew, monospaced size">Object</FONT>.</zparagraph>
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<A NAME="1193533"> </A>
<A NAME="Example"> </A>
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<zrefheader>Example</zrefheader>
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<A NAME="1193537"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>See <A HREF="object.html#1193628"  >watch</A>.</zparagraph>
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<A NAME="1193540"> </A>
<A NAME="valueOf"> </A>
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<zrefhead2>valueOf</zrefhead2>
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<A NAME="1193553"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Returns the primitive value of the specified object.<BR>
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<ztabletext><I>Method of</I></ztabletext>
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<A NAME="1193548"> </A>
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<ztabletext><A HREF="object.html#1193136"  >Object</A></ztabletext>
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<ztabletext><I>Implemented in</I></ztabletext>
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<A NAME="1193552"> </A>
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<ztabletext>JavaScript 1.1</ztabletext>
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<A NAME="1196506"> </A>
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<ztabletext><I>ECMA version</I></ztabletext>
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<A NAME="1196508"> </A>
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<ztabletext>ECMA-262</ztabletext>
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<A NAME="Syntax"> </A>
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<zrefheader>Syntax</zrefheader>
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<A NAME="1193555"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>valueOf()
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<A NAME="1193556"> </A>
<A NAME="Parameters"> </A>
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<zrefheader>Parameters</zrefheader>
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<A NAME="1193557"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>None</zparagraph>
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<A NAME="1200569"> </A>
<A NAME="Description"> </A>
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<zrefheader>Description</zrefheader>
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<A NAME="1204828"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>JavaScript calls the <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> method to convert an object to a primitive value. You rarely need to invoke the <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> method yourself; JavaScript automatically invokes it when encountering an object where a primitive value is expected.</zparagraph>
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<A NAME="1198170"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>By default, the <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> method is inherited by every object descended from <FONT FACE="courier, couriernew, monospaced size">Object</FONT>. Every built-in core object overrides this method to return an appropriate value. If an object has no primitive value, <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> returns the object itself, which is displayed as:</zparagraph>
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<A NAME="1198203"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>[object Object]
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<A NAME="1198132"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>You can use <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> within your own code to convert a built-in object into a primitive value. When you create a custom object, you can override <A HREF="object.html#1193540"  >Object.valueOf</A> to call a custom method instead of the default <FONT FACE="courier, couriernew, monospaced size">Object</FONT> method.</zparagraph>
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<A NAME="1193605"> </A>
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<zheadrunin>Overriding valueOf for custom objects. </zheadrunin>
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<A NAME="1198049"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>You can create a function to be called in place of the default <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> method. Your function must take no arguments.</zparagraph>
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<A NAME="1193606"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Suppose you have an object type <FONT FACE="courier, couriernew, monospaced size">myNumberType</FONT> and you want to create a <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> method for it. The following code assigns a user-defined function to the object's <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> method:</zparagraph>
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<A NAME="1193607"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>myNumberType.prototype.valueOf = new Function(functionText)
</zcodeline>
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<A NAME="1193608"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>With the preceding code in place, any time an object of type <FONT FACE="courier, couriernew, monospaced size">myNumberType</FONT> is used in a context where it is to be represented as a primitive value, JavaScript automatically calls the function defined in the preceding code.</zparagraph>
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<A NAME="1193609"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>An object's <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT> method is usually invoked by JavaScript, but you can invoke it yourself as follows:</zparagraph>
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<A NAME="1193610"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>myNumber.valueOf()
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<A NAME="1193611"> </A>
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<znote>Note</znote>
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<A NAME="1193615"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Objects in string contexts convert via the <A HREF="object.html#1193350"  >toString</A> method, which is different from <FONT FACE="courier, couriernew, monospaced size">String</FONT> objects converting to string primitives using <FONT FACE="courier, couriernew, monospaced size">valueOf</FONT>. All string objects have a string conversion, if only <FONT FACE="courier, couriernew, monospaced size">"[object </FONT><FONT FACE="courier, couriernew, monospaced size"><I>type</I></font><FONT FACE="courier, couriernew, monospaced size">]"</FONT>. But many objects do not convert to number, boolean, or function.</zparagraph>
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<A NAME="1193619"> </A>
<A NAME="See also"> </A>
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<zrefheader>See also</zrefheader>
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<A NAME="1193626"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><A HREF="toplev.html#1064173"  >parseInt</A>, <A HREF="object.html#1193350"  >Object.toString</A></zparagraph>
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<A NAME="1193628"> </A>
<A NAME="watch"> </A>
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<zrefhead2>watch</zrefhead2>
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<A NAME="1193630"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Watches for a property to be assigned a value and runs a function when that occurs.<BR>
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<A NAME="1193633"> </A>
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<ztabletext><I>Method of</I></ztabletext>
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<A NAME="1193638"> </A>
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<ztabletext><A HREF="object.html#1193136"  >Object</A></ztabletext>
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<A NAME="1193640"> </A>
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<ztabletext><I>Implemented in</I></ztabletext>
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<A NAME="1193642"> </A>
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<ztabletext>JavaScript 1.2, NES 3.0</ztabletext>
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<A NAME="1193643"> </A>
<A NAME="Syntax"> </A>
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<zrefheader>Syntax</zrefheader>
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<A NAME="1193644"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>watch(<I>prop</I>, <I>handler</I>)
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<A NAME="1193645"> </A>
<A NAME="Parameters"> </A>
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<zrefheader>Parameters</zrefheader>
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<A NAME="1193655"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><FONT FACE="courier, couriernew, monospaced size"><BR>
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<A NAME="1193648"> </A>
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<ztabletext><FONT FACE="courier, couriernew, monospaced size">prop</FONT></ztabletext>
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<A NAME="1193650"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>The name of a property of the object.</ztabletext>
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<ztabletext><FONT FACE="courier, couriernew, monospaced size">handler</FONT></ztabletext>
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<A NAME="1193654"> </A>
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<ztabletext>A function to call.</ztabletext>
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<A NAME="1193656"> </A>
<A NAME="Description"> </A>
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<zrefheader>Description</zrefheader>
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<A NAME="1193657"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Watches for assignment to a property named <FONT FACE="courier, couriernew, monospaced size">prop</FONT> in this object, calling <FONT FACE="courier, couriernew, monospaced size">handler(prop, oldval, newval)</FONT> whenever <FONT FACE="courier, couriernew, monospaced size">prop</FONT> is set and storing the return value in that property. A watchpoint can filter (or nullify) the value assignment, by returning a modified <FONT FACE="courier, couriernew, monospaced size">newval</FONT> (or <FONT FACE="courier, couriernew, monospaced size">oldval</FONT>).</zparagraph>
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<A NAME="1193658"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>If you delete a property for which a watchpoint has been set, that watchpoint does not disappear. If you later recreate the property, the watchpoint is still in effect. </zparagraph>
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<A NAME="1200696"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>To remove a watchpoint, use the <A HREF="object.html#1193499"  >unwatch</A> method. By default, the <FONT FACE="courier, couriernew, monospaced size">watch</FONT> method is inherited by every object descended from <FONT FACE="courier, couriernew, monospaced size">Object</FONT>.</zparagraph>
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<A NAME="1193663"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>The JavaScript debugger has functionality similar to that provided by this method, as well as other debugging options. For information on the debugger, see <I><A HREF="http://developer.netscape.com/docs/manuals/jsdebug/index.htm">Getting Started with Netscape JavaScript Debugger</A></I>.</zparagraph>
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<A NAME="1193669"> </A>
<A NAME="Example"> </A>
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<zrefheader>Example</zrefheader>
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<A NAME="1193670"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>&lt;script language="JavaScript1.2"&gt;<br>
o = {p:1}<br>
o.watch("p",<br>
&nbsp;&nbsp;&nbsp;function (id,oldval,newval) {<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;document.writeln("o." + id + " changed from " <br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ oldval + " to " + newval)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return newval<br>
&nbsp;&nbsp;&nbsp;})
</zcodeline>
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<P><A NAME="1193671"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>o.p = 2<br>
o.p = 3<br>
delete o.p<br>
o.p = 4
</zcodeline>
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<P><A NAME="1193672"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>o.unwatch('p')<br>
o.p = 5
</zcodeline>
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<P><A NAME="1193673"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>&lt;/script&gt;
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<A NAME="1193674"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>This script displays the following:</zparagraph>
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<A NAME="1193675"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>o.p changed from 1 to 2<br>o.p changed from 2 to 3<br>o.p changed from 3 to 4</zparagraph>
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