Check that the class has a component

[guilherme]

Hey, help me get this straight. It is not necessary to verify that the class is in the element and, in the absence of a class, add it. Here's the code:

    var cells = document.getElementsByTagName('td');
for(var i=0; i<cells.length;i++){
    cells[i].addEventListener('click', selectDate);
}
function selectDate(){
    if(cells.classList.contains('active')) {
        return false;
    }else{
        cells.classList.add('active');
    }
}

It's full code. http://jsfiddle.net/lilubanana/fwxuf5ea/22/

[Alberto]

selectDate - it's a processor's function, inside it. this points to the element on which they clicked. All you need to do is replace cells ♪ this

function selectDate(){
    if(this.classList.contains('active')) {
        return false;
    }else{
        this.classList.add('active');
    }
}

In addition, verification is not necessary in this case, as is classList.add doesn't add the class if he's already present.