JSはパラメータ関数の作成と呼び出しがあります.

function compute(op){
	var num1,num2;
	num1 = parseFloat(document.myForm.txtNum1.value);
	num2 = parseFloat(document.myForm.txtNum2.value);
	if(op=="+")
		document.myForm.txtResult.value = num1+num2;
	if(op=="-")
		document.myForm.txtResult.value = num1-num2;
	if(op=="*")
		document.myForm.txtResult.value = num1*num2;
	if(op=="/")
		document.myForm.txtResult.value = num1/num2;
	}