Simple Scientific Calculator using javascript

<script>
function addChar(input, character) {
	if(input.value == null || input.value == "0")
		input.value = character
	else
		input.value += character
}
 
function cos(form) {
	form.display.value = Math.cos(form.display.value);
}
 
function sin(form) {
	form.display.value = Math.sin(form.display.value);
}
 
function tan(form) {
	form.display.value = Math.tan(form.display.value);
}
 
function sqrt(form) {
	form.display.value = Math.sqrt(form.display.value);
}
 
function ln(form) {
	form.display.value = Math.log(form.display.value);
}
 
function exp(form) {
	form.display.value = Math.exp(form.display.value);
}
 
function deleteChar(input) {
	input.value = input.value.substring(0, input.value.length - 1)
}
 
function changeSign(input) {
	if(input.value.substring(0, 1) == "-")
		input.value = input.value.substring(1, input.value.length)
	else
		input.value = "-" + input.value
}
 
function compute(form) {
	form.display.value = eval(form.display.value)
}
 
function square(form) {
	form.display.value = eval(form.display.value) * eval(form.display.value)
}
 
function checkNum(str) {
	for (var i = 0; i < str.length; i++) {
		var ch = str.substring(i, i+1)
		if (ch < "0" || ch > "9") {
			if (ch != "/" && ch != "*" && ch != "+" && ch != "-" && ch != "."
				&& ch != "(" && ch!= ")") {
				alert("invalid entry!")
				return false
				}
			}
		}
		return true
}
</script>

	form{
	width:440px;
	padding:25px;
	margin:auto;
	border:5px solid #aaa;
	border-radius:15px 15px;
	}
 
	input{
	background:#888;
	color:#fff;
	font-size:15px;
	padding-top:15px;
	padding-bottom:15px;
	padding-left:19px;
	padding-right:19px;
	margin:6px;
	}
 
	h2{
	text-align:center;
	}
 
	.sc{
	background:#fff;
	color:#000;
	}

<form name="sci-calc">
<h2>SCIENTIFIC CALCULATOR</h2>
<table cellspacing="0" cellpadding="1">
<TR>
<TD COLSPAN="5" ALIGN="center"><input NAME="display" class="sc" VALUE="0" SIZE="44" MAXLENGTH="25"></TD>
</TR>
<TR>                                  
<td align="center"><input type="button" value="Clear" ONCLICK="this.form.display.value = 0 "></TD>
<td align="center" colspan="2"><input type="button" value="      Backspace     " ONCLICK="deleteChar(this.form.display)"></TD>
<td align="center" colspan="2"><input type="button" value="           Enter          " NAME="Enter" ONCLICK="if (checkNum(this.form.display.value)) { compute(this.form) }"></TD>
</TR> 
<TR>
<td align="center"><input type="button" value=" exp " ONCLICK="if (checkNum(this.form.display.value)) { exp(this.form) }"></TD>
<td align="center"><input type="button" value="  7  " ONCLICK="addChar(this.form.display, '7')"></TD>
<td align="center"><input type="button" value="  8  " ONCLICK="addChar(this.form.display, '8')"></TD>
<td align="center"><input type="button" value="  9  " ONCLICK="addChar(this.form.display, '9')"></TD>
<td align="center"><input type="button" value="   /   " ONCLICK="addChar(this.form.display, '/')"></TD>
</TR>                                  
<TR>                                   
<td align="center"><input type="button" value="  ln   " ONCLICK="if (checkNum(this.form.display.value)) { ln(this.form) }"></TD>
<td align="center"><input type="button" value="  4  " ONCLICK="addChar(this.form.display, '4')"></TD>
<td align="center"><input type="button" value="  5  " ONCLICK="addChar(this.form.display, '5')"></TD>
<td align="center"><input type="button" value="  6  " ONCLICK="addChar(this.form.display, '6')"></TD>
<td align="center"><input type="button" value="   *   " ONCLICK="addChar(this.form.display, '*')"></TD>
</TR>                                   
<TR>                                    
<td align="center"><input type="button" value=" sqrt " ONCLICK="if (checkNum(this.form.display.value)) { sqrt(this.form) }"></TD>
<td align="center"><input type="button" value="  1  " ONCLICK="addChar(this.form.display, '1')"></TD>
<td align="center"><input type="button" value="  2  " ONCLICK="addChar(this.form.display, '2')"></TD>
<td align="center"><input type="button" value="  3  " ONCLICK="addChar(this.form.display, '3')"></TD>
<td align="center"><input type="button" value="   -   " ONCLICK="addChar(this.form.display, '-')"></TD>
</TR>                                  
<TR>                                   
<td align="center"><input type="button" value="  sq  " ONCLICK="if (checkNum(this.form.display.value)) { square(this.form) }"></TD>
<td align="center"><input type="button" value="  0  " ONCLICK="addChar(this.form.display, '0')"></TD>
<td align="center"><input type="button" value="   .  " ONCLICK="addChar(this.form.display, '.')"></TD>
<td align="center"><input type="button" value=" +/- " ONCLICK="changeSign(this.form.display)"></TD>
<td align="center"><input type="button" value="   +  " ONCLICK="addChar(this.form.display, '+')"></TD>
</TR>                                  
<TR>                                   
<td align="center"><input type="button" value="   (    " ONCLICK="addChar(this.form.display, '(')"></TD>
<td align="center"><input type="button" value="cos" ONCLICK="if (checkNum(this.form.display.value)) { cos(this.form) }"></TD>
<td align="center"><input type="button" value=" sin " ONCLICK="if (checkNum(this.form.display.value)) { sin(this.form) }"></TD>
<td align="center"><input type="button" value=" tan" ONCLICK="if (checkNum(this.form.display.value)) { tan(this.form) }"></TD>
<td align="center"><input type="button" value="   )   " ONCLICK="addChar(this.form.display, ')')"></TD>
</TR>                                 
 
</table>
</form>