How to disable an input field using Javascript?

Question

I'm starting with Javascript, I wrote this function:

function disableField() {
  if( document.getElementById("valorFinal").length > 0 ) ) {
    document.getElementById("cantidadCopias").disabled = true; 
  }
}

Which disables the second field named cantidadCopias if the first one is filled.

<label> <span>Valor final:</span>
  <input type="text" class="input_text" name="valorFinal" id="valorFinal" onkeydown="disableField()"/>
</label>
<label> <span>Cantidad de Copias:</span>
  <input type="text" class="input_text" name="cantidadCopias" id="cantidadCopias"/>
</label>

But it's not disabling the second field when the first one is filled.

Answer 1

Did you look at the console?

• Uncaught SyntaxError: Unexpected token )
• Uncaught ReferenceError: disableField is not defined

First time you had a spelling error, now your code has an extra )

function disableField() {
  if( document.getElementById("valorFinal").length > 0 ) ) {  <-- extra )
    document.getElementById("cantidadCopias").disabled = true; 
  }
}​

Now the next issue is you are not looking at the length of the value.

if( document.getElementById("valorFinal").length > 0 )  <-- you are looking at the length of the HTML DOM Node.

So the code should look like

function disableField() {
  if( document.getElementById("valorFinal").value.length > 0 ) { 
    document.getElementById("cantidadCopias").disabled = true; 
  }
}​

but now how it is written, once it is disabled, it will not be re-enabled.

function disableField() {
    var isDisabled = document.getElementById("valorFinal").value.length > 0; 
    document.getElementById("cantidadCopias").disabled = isDisabled;
}​

Answer 2

It is best if you use onkeyup() instead of onkeydown(). The problem is the value of the input is not updated on keydown event.

Fiddle

<label> 
  <span>Valor final:</span>
  <input type="text" class="input_text" name="valorFinal" id="valorFinal" onkeyup="disableField(this.value)"/>
 </label>
<label> 
  <span>Cantidad de Copias:</span>
  <input type="text" class="input_text" name="cantidadCopias" id="cantidadCopias"/>
</label>

javascript

function disableField(val) {
    var cantidadCopias = document.getElementById("cantidadCopias");
    cantidadCopias.disabled = ( val.length > 0  );
}

Answer 3

the javascript:

var disableField = function () {
  var state = document.getElementById("valorFinal").value.length > 0;
  document.getElementById("cantidadCopias").disabled = state;
}​;​

the html:

<label> <span>Valor final:</span>
  <input type="text" class="input_text" name="valorFinal" id="valorFinal" onkeyup="disableField()"/>
</label>
<label> <span>Cantidad de Copias:</span>
  <input type="text" class="input_text" name="cantidadCopias" id="cantidadCopias"/>
</label>​

you should also enable it again when the input length is 0 again.

besides that you should hook onkeyup and not onkeydown.

you can try it here: jsfiddle.net/DBJfN/