javascript的計算機eval

<script language="javascript">

function compute(obj)

{obj.expr.value=eval(obj.expr.value)}

var one='1'

var two='2'

var three='3'

var four='4'

var five='5'

var six='6'

var seven='7'

var eight='8'

var nine='9'

var zero='0'

var plus='+'

var minus='-'

var multiply='*'

var divide='/'

var decimal='.'

function enter(obj,string)

{obj.expr.value+=string}

function clear(obj)

{obj.expr.value=' '}

</script>

<form name="calc">

<table border=1>

<td colspan=4><input type="text" name="expr" size=30 action="compute(this.form)"><tr>

<td><input type="button" value=" 7 " onClick="enter(this.form,seven)">

<td><input type="button" value=" 8 " onClick="enter(this.form,eight)">

<td><input type="button" value=" 9 " onClick="enter(this.form,nine)">

<td><input type="button" value=" / " onClick="enter(this.form,divide)">

<tr><td><input type="button" value=" 4 " onClick="enter(this.form,four)">

<td><input type="button" value=" 5 " onClick="enter(this.form,five)">

<td><input type="button" value=" 6 " onClick="enter(this.form,six)">

<td><input type="button" value=" * " onClick="enter(this.form,multiply)">

<tr><td><input type="button" value=" 1 " onClick="enter(this.form,one)">

<td><input type="button" value=" 2 " onClick="enter(this.form,two)">

<td><input type="button" value=" 3 " onClick="enter(this.form,three)">

<td><input type="button" value=" - " onClick="enter(this.form,minus)">

<tr><td colspan=2><input type="button" value=" 0 " onClick="enter(this.form,zero)">

<td><input type="button" value=" . " onClick="enter(this.form,decimal)">

<td><input type="button" value=" + " onClick="enter(this.form,plus)">

<tr><td colspan=2><input type="button" value="   =   " onClick="compute(this.form)">

<td colspan=2><input type="button" value="  AC  " size=3 onClick="clear"(this.form)">

</table>

</form>