number.html

javascript 1.5规范

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<A NAME="1206810"> </A>
<A NAME="Examples"> </A>
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<zrefheader>Examples</zrefheader>
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<A NAME="1206339"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>var num=77.1234<br>
alert("num.toExponential() is " + num.toExponential()) //displays 
7.71234e+1<br>
alert("num.toExponential(4) is " + num.toExponential(4)) //displays 7.7123e+1&nbsp;&nbsp;&nbsp;<br>
alert("num.toExponential(2) is " + num.toExponential(2)) //displays 
7.71e+1<br>
alert("77.1234.toExponential() is " + 77.1234.toExponential())<br>
//displays 7.71234e+1<br>
alert("77 .toExponential() is " + 77 .toExponential()) //displays 7.7e+1
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<A NAME="1201287"> </A>
<A NAME="See also"> </A>
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<zrefheader>See also</zrefheader>
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<A NAME="1201291"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><A HREF="number.html#1200964"  >toFixed</A>, <A HREF="number.html#1201389"  >toPrecision</A>, <A HREF="number.html#1193464"  >toString</A></zparagraph>
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<A NAME="1200964"> </A>
<A NAME="toFixed"> </A>
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<zrefhead2>toFixed</zrefhead2>
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<A NAME="1200232"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Returns a string representing the Number object in fixed-point notation.<BR>
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<A NAME="1200218"> </A>
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<ztabletext><I>Method of</I></ztabletext>
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<A NAME="1200223"> </A>
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<ztabletext><A HREF="number.html#1193137"  >Number</A></ztabletext>
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<A NAME="1200225"> </A>
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<ztabletext><I>Implemented in</I></ztabletext>
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<A NAME="1200227"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>JavaScript 1.5</ztabletext>
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<A NAME="1200229"> </A>
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<ztabletext><I>ECMA version</I></ztabletext>
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<A NAME="1200231"> </A>
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<ztabletext>ECMA-262, Edition 3</ztabletext>
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<P> </zparagraph>
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<A NAME="1200233"> </A>
<A NAME="Syntax"> </A>
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<zrefheader>Syntax</zrefheader>
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<A NAME="1200234"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>toFixed([<I>fractionDigits</I>])
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<A NAME="1200235"> </A>
<A NAME="Parameters"> </A>
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<zrefheader>Parameters</zrefheader>
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<A NAME="1200241"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><BR>
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    <TD VALIGN=TOP><A NAME="1200341"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>fractionDigits
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<A NAME="1200343"> </A>
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<ztabletext>An integer specifying the number of digits after the decimal point. Defaults to zero.</ztabletext>
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<A NAME="1200242"> </A>
<A NAME="Description"> </A>
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<zrefheader>Description</zrefheader>
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<A NAME="1200525"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>The <FONT FACE="courier, couriernew, monospaced size">Number.prototype.toFixed</FONT> method returns a string representing a Number object in fixed-point notation, rounded to the number of digits after the decimal point specified by <FONT FACE="courier, couriernew, monospaced size"><I>fractionDigits</I></font>. </zparagraph>
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<A NAME="1201067"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>The output of <FONT FACE="courier, couriernew, monospaced size">toFixed</FONT> may be more precise than <FONT FACE="courier, couriernew, monospaced size">toString</FONT> for some values, because <FONT FACE="courier, couriernew, monospaced size">toString</FONT> outputs only enough significant digits to distinguish the number from adjacent number values.</zparagraph>
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<A NAME="1207032"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>If a number has more digits that requested by the <FONT FACE="courier, couriernew, monospaced size"><I>fractionDigits</I></font> parameter, the number is rounded to the nearest number represented by <FONT FACE="courier, couriernew, monospaced size"><I>fractionDigits </I></font>digits. If the number is exactly halfway between two representable numbers, it is rounded away from zero (up if it is positive, down if it is negative). Thus:</zparagraph>
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<dl>  <dl>
     <DT> <A NAME="1207136"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<zparaindent1><FONT FACE="courier, couriernew, monospaced size">0.124.toFixed(2)</FONT> returns "0.12".</zparaindent1>
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<P>     <DT> <A NAME="1207163"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<zparaindent1><FONT FACE="courier, couriernew, monospaced size">0.125.tofixed(2) </FONT>returns "0.13", because 0.125 is exactly halfway between 0.12 and 0.13.</zparaindent1>
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<P>     <DT> <A NAME="1207201"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<zparaindent1>0<FONT FACE="courier, couriernew, monospaced size">.126.tofixed(2)</FONT> returns "0.13".</zparaindent1>
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<A NAME="1207214"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Given this convention, one might expect <FONT FACE="courier, couriernew, monospaced size">0.045.toFixed(2)</FONT> to return "0.05", but it returns "0.04". This is because of the way computers represent IEEE 754 floating-point numbers. The IEEE 754 standard uses binary fractions (fractions of 0's and 1's after the dot). Just as some numbers, such as 1/3, are not representable precisely as decimal fractions, other numbers, such as 0.045, are not precisely representable as binary fractions. The IEEE 754 standard dictates that 0.045 be approximated to 0.04499999999999999833466546306226518936455249786376953125, which is precisely representable as a binary fraction. This approximation is closer to 0.04 than to 0.05,<FONT FACE="courier, couriernew, monospaced size"> </FONT>so <FONT FACE="courier, couriernew, monospaced size">0.045.toFixed(2)</FONT> returns "0.04".</zparagraph>
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<BR>
<A NAME="1201068"> </A>
<A NAME="Examples"> </A>
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<zrefheader>Examples</zrefheader>
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<A NAME="1200863"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>var num=10.1234<br>
alert("num.toFixed() is " + num.toFixed()) //displays 10<br>
alert("num.toFixed(4) is " + num.toFixed(4)) //displays 10.1234&nbsp;&nbsp;&nbsp;
alert("num.toFixed(2) is " + num.toFixed(2)) //displays 10.12
</zcodeline>
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<P><BR>
<A NAME="1201348"> </A>
<A NAME="See also"> </A>
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<zrefheader>See also</zrefheader>
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<A NAME="1201355"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><A HREF="number.html#1200968"  >toExponential</A>, <A HREF="number.html#1201389"  >toPrecision</A>, <A HREF="number.html#1193464"  >toString</A></zparagraph>
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<A NAME="1201389"> </A>
<A NAME="toPrecision"> </A>
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<zrefhead2>toPrecision</zrefhead2>
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<A NAME="1201406"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>Returns a string representing the Number object to the specified precision.<BR>
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<A NAME="1201392"> </A>
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<ztabletext><I>Method of</I></ztabletext>
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<A NAME="1201397"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><A HREF="number.html#1193137"  >Number</A></ztabletext>
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<A NAME="1201399"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><I>Implemented in</I></ztabletext>
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<A NAME="1201401"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>JavaScript 1.5</ztabletext>
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<A NAME="1201403"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext><I>ECMA version</I></ztabletext>
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<A NAME="1201405"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>ECMA-262, Edition 3</ztabletext>
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<P> </zparagraph>
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<BR>
<A NAME="1201407"> </A>
<A NAME="Syntax"> </A>
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<zrefheader>Syntax</zrefheader>
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<A NAME="1201408"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>toPrecision([<I>precision</I>])
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<A NAME="1201409"> </A>
<A NAME="Parameters"> </A>
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<zrefheader>Parameters</zrefheader>
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<A NAME="1201415"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph><BR>
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<zcodeline>precision
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<A NAME="1201414"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
<ztabletext>An integer specifying the number of digits after the decimal point. </ztabletext>
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<BR>
<A NAME="1201416"> </A>
<A NAME="Description"> </A>
<FONT FACE="Arial, Helvetica, sans-serif" SIZE="-1">
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<zrefheader>Description</zrefheader>
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<A NAME="1201719"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>The <FONT FACE="courier, couriernew, monospaced size">Number.prototype.toPrecision</FONT> method returns a string representing a Number object in fixed-point or exponential notation rounded to <FONT FACE="courier, couriernew, monospaced size"><I>precision</I></font> significant digits.</zparagraph>
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<A NAME="1206997"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>If you use the <FONT FACE="courier, couriernew, monospaced size">toPrecision</FONT> method for a numeric literal and the numeric literal has no exponent and no decimal point, leave a space before the dot that precedes the method call to prevent the dot from being interpreted as a decimal point.</zparagraph>
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<A NAME="1201723"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>If the <FONT FACE="courier, couriernew, monospaced size"><I>precision</I></font> argument is omitted, behaves as <FONT FACE="courier, couriernew, monospaced size">Number.prototype.toString</FONT>.</zparagraph>
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<A NAME="1207425"> </A><FONT FACE="Arial, Helvetica, sans-serif" SIZE="2"><zparagraph>If a number has more digits that requested by the <FONT FACE="courier, couriernew, monospaced size"><I>precision </I></font>parameter, the number is rounded to the nearest number represented by <FONT FACE="courier, couriernew, monospaced size"><I>precision </I></font>digits. See the discussion of rounding in the description of the <FONT FACE="courier, couriernew, monospaced size">toFixed</FONT> method on <A HREF="number.html#1207032"  >page&nbsp;129</A>, which also applies to <FONT FACE="courier, couriernew, monospaced size">toPrecision</FONT>.</zparagraph>
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<A NAME="1201927"> </A>
<A NAME="Examples"> </A>
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<zrefheader>Examples</zrefheader>
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<A NAME="1202066"></A><FONT FACE="courier, couriernew, monospaced size" SIZE="-1">
<zcodeline>var num=5.123456<br>
alert("num.toPrecision() is " + num.toPrecision()) //displays 5.123456<br>
alert("num.toPrecision(4) is " + num.toPrecision(4)) //displays 5.123<br>
alert("num.toPrecision(2) is " + num.toPrecision(2)) //displays 5.1<br>
alert("num.toPrecision(2) is " + num.toPrecision(1)) //displays 5<br>
alert("num.toPrecision(2) is " + num.toPrecision(1)) //displays 5<br>
alert("1250 .toPrecision() is " + 1250 .toPrecision(2))<br>
//displays 1.3e+3<br>
alert("1250 .toPreci