Multiple if statements and then else Javascript
I'm trying to create a calculator in javascript for inverse trigonometry functions (arcsine, arccosine, arctangent) and I have it so there are checkboxes for each input so window.alert doesn't return NULL (user-friendly).
The Form:

    <form name="inverse">
			<legend>Legend is here.</legend>
			<input type="checkbox" name="inverse-cb1" value="sine"></input><label> sin<sup>-1</sup> ( </label><input type="text" size=5 name="sin"><label> )</label><br>
			<input type="checkbox" name="inverse-cb2" value="cosine"></input><label> cos<sup>-1</sup> ( </label><input type="text" size=5 name="cos"><label> )</label><br>
			<input type="checkbox" name="inverse-cb3" value="tangent"></input><label> tan<sup>-1</sup> ( </label><input type="text" size=5 name="tan"><label> )</label><br>
			<br><input type="button" onclick="trigI()" value="Calculate">
		</form>

The script:

    function trigI(){
    var sin = document.inverse.sin.value;      //sine
    var cos = document.inverse.cos.value;      //cosine
    var tan = document.inverse.tan.value;      //tangent
	
	var sin1 = Math.asin(sin);      //arcsine
    var cos1 = Math.acos(cos);      //arccosine
    var tan1 = Math.atan(tan);      //arctangent
	
    var sin1d = sin1 * (180/Math.PI);    //convert radians to degrees (sine)
    var cos1d = cos1 * (180/Math.PI);    //convert radians to degrees (cosine)
	var tan1d = tan1 * (180/Math.PI);    //convert radians to degrees (tangent)
	
	var sup = -1;       //superscript
	
	if (isNaN(document.inverse.sin.value) == true || isNaN(document.inverse.cos.value) == true || isNaN(document.inverse.tan.value) == true){
        window.alert("Please input a number.");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
    }
    
    if (document.inverse-cb1.checked == 'false'){    //no sine input
		window.alert("When cos(\u2220) = " + cos + " and tan(\u2220) = " + tan + " :" + "\n\n" + "cos" + sup.sup() + "(A) = " + cos1d + "\n" + "tan" + sup.sup() + "(A) = " + tan1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb2.checked == 'false'){    //no cosine input
		window.alert("When sin(\u2220) = " + sin + " and tan(\u2220) = " + tan + " :" + "\n\n" + "sin" + sup.sup() + "(A) = " + sin1d + "\u00B0" + "\n" + "tan" + sup.sup() + "(A) = " + tan1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb3.checked == 'false'){    //no tangent input
		window.alert("When sin(\u2220) = " + sin + " and cos(\u2220) = " + cos + " :" + "\n\n" + "sin" + sup.sup() + "(A) = " + sin1d + "\u00B0" + "\n" + "cos" + sup.sup() + "(A) = " + cos1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb1.checked == 'false' && document.inverse-cb2.checked == 'false'){       //no sine and cosine input
		window.alert("When tan(\u2220) = " + tan + " :" + "\n\n" + "tan" + sup.sup() + "(A) = " + tan1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb2.checked == 'false' && document.inverse-cb3.checked == 'false'){       //no cosine and tangent input
		window.alert("When sin(\u2220) = " + sin + " :" + "\n\n" + "sin" + sup.sup() + "(A) = " + sin1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb1.checked == 'false' && document.inverse-cb3.checked == 'false'){       //no sine and tangent input
		window.alert("When cos(\u2220) = " + cos + " :" + "\n\n" + "cos" + sup.sup() + "(A) = " + cos1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb1.checked == 'false' && document.inverse-cb2.checked == 'false' && document.inverse-cb3.checked == 'false'){     //no input or checked boxes
		window.alert("Please input a number or check the correct boxes.");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	else {
		window.alert("When sin(\u2220) = " + sin + ", cos(\u2220) = " + cos + ", and tan(\u2220) = " + tan + " :" + "\n\n" + "sin" + sup.sup() + "(A) = " + sin1d + "\u00B0" + "\n" + "cos" + sup.sup() + "(A) = " + cos1d + "\u00B0" + "\n" + "tan" + sup.sup() + "(A) = " + tan1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
}


Multiple if statements and then else Javascript
===

I'm trying to create a calculator in javascript for inverse trigonometry functions (arcsine, arccosine, arctangent) and I have it so there are checkboxes for each input so window.alert doesn't return NULL (user-friendly).
The Form:

    <form name="inverse">
			<legend>Legend is here.</legend>
			<input type="checkbox" name="inverse-cb1" value="sine"></input><label> sin<sup>-1</sup> ( </label><input type="text" size=5 name="sin"><label> )</label><br>
			<input type="checkbox" name="inverse-cb2" value="cosine"></input><label> cos<sup>-1</sup> ( </label><input type="text" size=5 name="cos"><label> )</label><br>
			<input type="checkbox" name="inverse-cb3" value="tangent"></input><label> tan<sup>-1</sup> ( </label><input type="text" size=5 name="tan"><label> )</label><br>
			<br><input type="button" onclick="trigI()" value="Calculate">
		</form>

The script:

    function trigI(){
    var sin = document.inverse.sin.value;      //sine
    var cos = document.inverse.cos.value;      //cosine
    var tan = document.inverse.tan.value;      //tangent
	
	var sin1 = Math.asin(sin);      //arcsine
    var cos1 = Math.acos(cos);      //arccosine
    var tan1 = Math.atan(tan);      //arctangent
	
    var sin1d = sin1 * (180/Math.PI);    //convert radians to degrees (sine)
    var cos1d = cos1 * (180/Math.PI);    //convert radians to degrees (cosine)
	var tan1d = tan1 * (180/Math.PI);    //convert radians to degrees (tangent)
	
	var sup = -1;       //superscript
	
	if (isNaN(document.inverse.sin.value) == true || isNaN(document.inverse.cos.value) == true || isNaN(document.inverse.tan.value) == true){
        window.alert("Please input a number.");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
    }
    
    if (document.inverse-cb1.checked == 'false'){    //no sine input
		window.alert("When cos(\u2220) = " + cos + " and tan(\u2220) = " + tan + " :" + "\n\n" + "cos" + sup.sup() + "(A) = " + cos1d + "\n" + "tan" + sup.sup() + "(A) = " + tan1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb2.checked == 'false'){    //no cosine input
		window.alert("When sin(\u2220) = " + sin + " and tan(\u2220) = " + tan + " :" + "\n\n" + "sin" + sup.sup() + "(A) = " + sin1d + "\u00B0" + "\n" + "tan" + sup.sup() + "(A) = " + tan1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb3.checked == 'false'){    //no tangent input
		window.alert("When sin(\u2220) = " + sin + " and cos(\u2220) = " + cos + " :" + "\n\n" + "sin" + sup.sup() + "(A) = " + sin1d + "\u00B0" + "\n" + "cos" + sup.sup() + "(A) = " + cos1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb1.checked == 'false' && document.inverse-cb2.checked == 'false'){       //no sine and cosine input
		window.alert("When tan(\u2220) = " + tan + " :" + "\n\n" + "tan" + sup.sup() + "(A) = " + tan1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb2.checked == 'false' && document.inverse-cb3.checked == 'false'){       //no cosine and tangent input
		window.alert("When sin(\u2220) = " + sin + " :" + "\n\n" + "sin" + sup.sup() + "(A) = " + sin1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb1.checked == 'false' && document.inverse-cb3.checked == 'false'){       //no sine and tangent input
		window.alert("When cos(\u2220) = " + cos + " :" + "\n\n" + "cos" + sup.sup() + "(A) = " + cos1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	if (document.inverse-cb1.checked == 'false' && document.inverse-cb2.checked == 'false' && document.inverse-cb3.checked == 'false'){     //no input or checked boxes
		window.alert("Please input a number or check the correct boxes.");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
	
	else {
		window.alert("When sin(\u2220) = " + sin + ", cos(\u2220) = " + cos + ", and tan(\u2220) = " + tan + " :" + "\n\n" + "sin" + sup.sup() + "(A) = " + sin1d + "\u00B0" + "\n" + "cos" + sup.sup() + "(A) = " + cos1d + "\u00B0" + "\n" + "tan" + sup.sup() + "(A) = " + tan1d + "\u00B0");
		
		var sin = null;     //reset
		var cos = null;     //reset
		var tan = null;     //reset
		
		var sin1 = null;    //reset
		var cos1 = null;    //reset
		var tan1 = null;    //reset
		
		var sin1d = null;   //reset
		var cos1d = null;   //reset
		var tan1d = null;   //reset
		
		var sup = null;    //reset
		
		return;
	}
}

