number.html

javascript 1.5规范

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<A NAME="1193285"> </A>
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<ztabletext><I>Static, Read-only</I></ztabletext>
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<A NAME="1193289"> </A>
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<ztabletext><I>Implemented in</I></ztabletext>
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<A NAME="1193291"> </A>
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<ztabletext>JavaScript 1.1, NES 2.0</ztabletext>
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<A NAME="1194471"> </A>
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<ztabletext><I>ECMA version</I></ztabletext>
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<A NAME="1194473"> </A>
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<ztabletext>ECMA-262</ztabletext>
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<A NAME="1193293"> </A>
<A NAME="Description"> </A>
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<zrefheader>Description</zrefheader>
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<A NAME="1196879"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>The <FONT FACE="courier, couriernew, monospaced size">MIN_VALUE</FONT> property is the number closest to 0, not the most negative number, that JavaScript can represent.</zparagraph>
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<A NAME="1196880"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><FONT FACE="courier, couriernew, monospaced size">MIN_VALUE</FONT> has a value of approximately 5e-324. Values smaller than <FONT FACE="courier, couriernew, monospaced size">MIN_VALUE</FONT> ("underflow values") are converted to 0.</zparagraph>
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<A NAME="1193296"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Because <FONT FACE="courier, couriernew, monospaced size">MIN_VALUE</FONT> is a static property of <FONT FACE="courier, couriernew, monospaced size">Number</FONT>, you always use it as <FONT FACE="courier, couriernew, monospaced size">Number.MIN_VALUE</FONT>, rather than as a property of a <FONT FACE="courier, couriernew, monospaced size">Number</FONT> object you created.</zparagraph>
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<A NAME="1193297"> </A>
<A NAME="Examples"> </A>
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<zrefheader>Examples</zrefheader>
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<A NAME="1193298"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>The following code divides two numeric values. If the result is greater than or equal to <FONT FACE="courier, couriernew, monospaced size">MIN_VALUE</FONT>, the <FONT FACE="courier, couriernew, monospaced size">func1</FONT> function is called; otherwise, the <FONT FACE="courier, couriernew, monospaced size">func2</FONT> function is called.</zparagraph>
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<A NAME="1193299"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>if (num1 / num2 &gt;= Number.MIN_VALUE)<br>
&nbsp;&nbsp;&nbsp;func1()<br>
else<br>
&nbsp;&nbsp;&nbsp;func2()
</zcodeline>
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<A NAME="1193302"> </A>
<A NAME="NaN"> </A>
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<zrefhead2>NaN</zrefhead2>
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<A NAME="1193303"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>A special value representing Not-A-Number. This value is represented as the unquoted literal NaN.<BR>
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<A NAME="1193306"> </A>
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<ztabletext><I>Property of</I></ztabletext>
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<A NAME="1193311"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><A HREF="number.html#1193137"  >Number</A></ztabletext>
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<A NAME="1193313"> </A>
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<ztabletext><I> Read-only</I></ztabletext>
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<A NAME="1193315"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext></ztabletext>
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<A NAME="1193317"> </A>
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<ztabletext><I>Implemented in</I></ztabletext>
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<A NAME="1193319"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>JavaScript 1.1, NES 2.0</ztabletext>
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<A NAME="1194494"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><I>ECMA version</I></ztabletext>
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<A NAME="1194496"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>ECMA-262</ztabletext>
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<A NAME="1193320"> </A>
<A NAME="Description"> </A>
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<zrefheader>Description</zrefheader>
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<A NAME="1193321"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>JavaScript prints the value <FONT FACE="courier, couriernew, monospaced size">Number.NaN</FONT> as <FONT FACE="courier, couriernew, monospaced size">NaN</FONT>.</zparagraph>
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<A NAME="1193322"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><FONT FACE="courier, couriernew, monospaced size">NaN</FONT> is always unequal to any other number, including NaN itself; you cannot check for the not-a-number value by comparing to <FONT FACE="courier, couriernew, monospaced size">Number.NaN</FONT>. Use the <B><A HREF="toplev.html#1064024"  >isNaN</A></B> function instead.</zparagraph>
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<A NAME="1193326"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>You might use the <FONT FACE="courier, couriernew, monospaced size">NaN</FONT> property to indicate an error condition for a function that should return a valid number.</zparagraph>
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<A NAME="1193327"> </A>
<A NAME="Examples"> </A>
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<zrefheader>Examples</zrefheader>
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<A NAME="1193328"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>In the following example, if <FONT FACE="courier, couriernew, monospaced size">month</FONT> has a value greater than 12, it is assigned NaN, and a message is displayed indicating valid values.</zparagraph>
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<A NAME="1193329"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>var month = 13<br>
if (month &lt; 1 || month &gt; 12) {<br>
&nbsp;&nbsp;&nbsp;month = Number.NaN<br>
&nbsp;&nbsp;&nbsp;alert("Month must be between 1 and 12.")<br>
}
</zcodeline>
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<A NAME="1193330"> </A>
<A NAME="See also"> </A>
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<zrefheader>See also</zrefheader>
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<A NAME="1193343"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><FONT FACE="courier, couriernew, monospaced size"><A HREF="toplev.html#1064064"  >NaN</A></FONT>, <A HREF="toplev.html#1064024"  >isNaN</A>, <A HREF="toplev.html#1064132"  >parseFloat</A>, <A HREF="toplev.html#1064173"  >parseInt</A> </zparagraph>
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<A NAME="1193346"> </A>
<A NAME="NEGATIVE_INFINITY"> </A>
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<zrefhead2>NEGATIVE_INFINITY</zrefhead2>
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<A NAME="1193347"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>A special numeric value representing negative infinity. This value is represented as the unquoted literal <FONT FACE="courier, couriernew, monospaced size">"-Infinity"</FONT>.<BR>
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<A NAME="1193350"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><I>Property of</I></ztabletext>
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<A NAME="1193355"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><A HREF="number.html#1193137"  >Number</A></ztabletext>
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<A NAME="1193357"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><I>Static, Read-only</I></ztabletext>
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<A NAME="1193361"> </A>
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<ztabletext><I>Implemented in</I></ztabletext>
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<A NAME="1193363"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>JavaScript 1.1, NES 2.0</ztabletext>
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<A NAME="1194499"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><I>ECMA version</I></ztabletext>
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<A NAME="1194501"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>ECMA-262</ztabletext>
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<A NAME="1193364"> </A>
<A NAME="Description"> </A>
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<zrefheader>Description</zrefheader>
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<A NAME="1196964"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>This value behaves slightly differently than mathematical infinity:</zparagraph>
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<P>
<ul>
<A NAME="1197134"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<B>  <li></B>
<zsmartlist1>Any positive value, including <FONT FACE="courier, couriernew, monospaced size">POSITIVE_INFINITY</FONT>, multiplied by <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT> is <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT>.</zsmartlist1>
</FONT>
<P><A NAME="1197135"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<B>  <li></B>
<zsmartlist1>Any negative value, including <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT>, multiplied by <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT> is <FONT FACE="courier, couriernew, monospaced size">POSITIVE_INFINITY</FONT>.</zsmartlist1>
</FONT>
<P><A NAME="1197109"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<B>  <li></B>
<zsmartlist1>Zero multiplied by <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT> is <FONT FACE="courier, couriernew, monospaced size">NaN</FONT>.</zsmartlist1>
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<P><A NAME="1197304"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<B>  <li></B>
<zsmartlist1><FONT FACE="courier, couriernew, monospaced size">NaN</FONT> multiplied by <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT> is <FONT FACE="courier, couriernew, monospaced size">NaN</FONT>.</zsmartlist1>
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<P><A NAME="1197305"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<B>  <li></B>
<zsmartlist1><FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT>, divided by any negative value except <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT>, is <FONT FACE="courier, couriernew, monospaced size">POSITIVE_INFINITY</FONT>.</zsmartlist1>
</FONT>
<P><A NAME="1197467"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<B>  <li></B>
<zsmartlist1><FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT>, divided by any positive value except <FONT FACE="courier, couriernew, monospaced size">POSITIVE_INFINITY</FONT>, is <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT>.</zsmartlist1>
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<P><A NAME="1197441"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<B>  <li></B>
<zsmartlist1><FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT>, divided by either <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT> or <FONT FACE="courier, couriernew, monospaced size">POSITIVE_INFINITY</FONT>, is <FONT FACE="courier, couriernew, monospaced size">NaN</FONT>.</zsmartlist1>
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<P><A NAME="1197517"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<B>  <li></B>
<zsmartlist1>Any number divided by <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT> is Zero.</zsmartlist1>
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<P></ul>

<A NAME="1193366"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Because <FONT FACE="courier, couriernew, monospaced size">NEGATIVE_INFINITY</FONT> is a static property of <FONT FACE="courier, couriernew, monospaced size">Number</FONT>, you always use it as <FONT FACE="courier, couriernew, monospaced size">Number.NEGATIVE_INFINITY</FONT>, rather than as a property of a <FONT FACE="courier, couriernew, monospaced size">Number</FONT> object you created.</zparagraph>
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<A NAME="1193367"> </A>
<A NAME="Examples"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
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<zrefheader>Examples</zrefheader>
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<A NAME="1193368"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>In the following example, the variable <FONT FACE="courier, couriernew, monospaced size">smallNumber</FONT> is assigned a value that is smaller than the minimum value. When the <FONT FACE="courier, couriernew, monospaced size">if</FONT> statement executes, <FONT FACE="courier, couriernew, monospaced size">smallNumber</FONT> has the value <FONT FACE="courier, couriernew, monospaced size">"-Infinity"</FONT>, so the <FONT FACE="courier, couriernew, monospaced size">func1</FONT> function is called.</zparagraph>
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<A NAME="1193369"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>var smallNumber = -Number.MAX_VALUE*10<br>
if (smallNumber == Number.NEGATIVE_INFINITY)<br>
&nbsp;&nbsp;&nbsp;func1()<br>
else<br>
&nbsp;&nbsp;&nbsp;func2()
</zcodeline>
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<A NAME="1193370"> </A>
<A NAME="See also"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
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