Actualizar Variables sin Recargar

[IsaBelM]

prueba con esto

function multiplicar(op2) {
console.log('valor operando2 ' + op2)
Array.prototype.map.call(document.querySelectorAll ('input[name="operando1"]'), function(op1) {

console.log(op1.value * op2);
})
}


<form name="calc">
<input type="text" name="operando1" value="2">
<input type="text" name="operando1" value="4">
<input type="text" name="operando1" value="6">
<select name="operando2" onchange="multiplicar(this.value)">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
</form>