Validating text box

Always when I do a javascript validation of a text box, to see if it is empty or not, I do it as follows:

function validar(){
    var input = document.getElementById("texto");
    if(input.value == ""){

         alert("Preencha todos os campos em branco.")

    }
}

However, today I discovered that this has a flaw, if the user leaves a space like this in the text box:

...andvalidatetheformwillnotrecognizeitasempty.

Whileitmightbepossibletodothis:

functionvalidar(){varinput=document.getElementById("texto");
    if((input.value == "") || (input.value == " ")){
         alert("Preencha todos os campos em branco.")
    }
}

.. And knowing that there are such cool solutions, I need to know if you can check for any characters inside the text box. And a way to do it in pure javascript.