change variable with onclick function

Question

I am trying to make a function that will:

run through all of the checkboxes I have checked in the html document
only allow 'checkboxLimit' to be checked (i.e. 4 checkboxes out of the 10 can be checked in the document)
return an array of the id's of the checked checkboxes (which I call cbl)

Currently, my issue has something to do with the scope of the onclick function.

function getCheckboxes(checkboxLimit) {
    // clear cbl
    cbl = ['','','',''];

    // contstuct list of checkbox tags 
    inputs = document.getElementsByTagName('input');
    var checkboxes = [];
    for ( var i = 0 ; i < inputs.length ; i++ ) {
        if (inputs[i].type == 'checkbox') 
        checkboxes.push(inputs[i]);
    }

All of the code above is functional, but the code below is where I run into issues. The function that is executed after 'onclick' works nicely, because if I alert cbl in the function, it works as I like (shown below). However, once cbl is alerted after the 'onclick' function, it is no longer the same.

    // construct list of checked checkboxes (limited)
    for ( var i = 0 ; i < checkboxes.length ; i++ ) {
        // if any checkbox is clicked
        checkboxes[i].onclick = function() {
            var checkCount = 0;
            // run through each of the checkboxes
            for ( var j = 0 ; j < checkboxes.length ; j++ ) {
                // if index checkbox is checked
                if ( checkboxes[j].checked == true ) {
                    // add to count
                    checkCount++;
                    // if count is above limit, uncheck
                    // otherwise add to list        
                    if ( checkCount > checkboxLimit)
                        this.checked = false;
                    else
                        cbl[checkCount-1] = checkboxes[j].id;
                }
            }
            // alert that displays cbl how I want
            alert(cbl);
        }
        // alert that does not display cbl how I want
        alert(cbl);
    }
}

So is there some way I can get past this scope issue? I would prefer staying away from JQuery, but whatever can get me to have functional code will work at this point.

I am unable to understand the checkboxes[i].onclick. Why is that in a loop? Have you tried putting a custom class like class='mycheckbox' and binding the onclick function to that? Looping through checkboxes and binding function seems odd. — sbjumani
– sbjumani, Commented Sep 21, 2014 at 23:09
A reference of how it is done with buttons. Can be used on checkboxes too. stackoverflow.com/questions/13818839/… — sbjumani
– sbjumani, Commented Sep 21, 2014 at 23:11
what do you expect cbl to be when it doesn't display how you want ? — ShaneQful
– ShaneQful, Commented Sep 21, 2014 at 23:13
So the first alert shows the id's of the checkboxes that are checked. For example: checkbox1,checkbox2,checkbox3,checkbox4 Whereas the function outside of the onclick is alerting the cleared ['','','',''] that I assigned in the first line of the function. So as it works right now, the cbl in the function is not affected at all by onclick. — mvarble
– mvarble, Commented Sep 21, 2014 at 23:22
You would need a closure inside your loop like (function(i){checkbokes[i].onclick = function(){}})(i); or you'll be at the end of your loop, but that's not the behavior you're looking for anyways. I'm going to make you a function that will allow you to test for a max and minimum. Hold on. — StackSlave
– StackSlave, Commented Sep 21, 2014 at 23:23

StackSlave · Accepted Answer · 2014-09-22 00:12:20Z

Try something more like this:

function checkboxValidate(collection, min, max){
  var mn = typeof min === 'undefined' ? 0 : min;
  var mx = typeof max === 'undefined' ? Infinity : max;
  var cb = [], n = 0;
  for(var i=0,l=collection.length; i<l, i++){
    var c = collection[i];
    if(c.type === 'checkbox' && c.checked){
      cb.push(c); n++;
    }
  }
  if(n < mn){
    // somthing.innerHTML = 'Minimum Requirement Failure';
    return false;
  }
  else if(n > mx){
    // somthing.innerHTML = 'Maximum Requirement Failure';
    return false;
  }
  else{
    return cb;
  }
}
anotherElement.onclick = function(){
  var checkedArray = checkboxValidate(document.getElementsByTagName('input'), 0, 3));
  if(checkedArray && checkedArray[0]){
    // checkedArray has each checkbox Element - more properties than just `id`
  }
  else{
    // checkedArray would be false or not have any checked with min 0
  }
}

Adjust as needed.

user13500 · Accepted Answer · 2014-09-22 00:03:42Z

A non cross-browser crude sample. Perhaps you can get something from it:

Fiddle

function limitCheck(n) {
    var x = [];
    document.addEventListener('click', function(e) {
        var i, t = e.target;
        if (t.type === 'checkbox') {
            if ((i = x.indexOf(t.id)) > -1) {
                console.log('remove ' + i + ', ' + t.id);
                x.splice(i, 1);
            } else if (x.length < n) {
                x.push(t.id);
            } else {
                console.log('LIM');
                e.preventDefault();
                e.stopPropagation();
            }
            console.log(x);
        }
    });
} 

limitCheck(4);

Without debug:

function limitCheck(n) {
    var x = [];
    document.addEventListener('click', function(e) {
        var i, t = e.target;
        if (t.type === 'checkbox') {
            if ((i = x.indexOf(t.id)) > -1) {
                x.splice(i, 1);
            } else if (x.length < n) {
                x.push(t.id);
            } else {
                e.preventDefault();
                e.stopPropagation();
            }
        }
    });
}

RobG · Accepted Answer · 2014-09-22 00:55:38Z

You can put a click listener on an ancestor of the checkboxes, probably they're in a form so use that. You also need to account for a reset button, and for buttons that are checked by default (so you also need to run the function on page load to pre-populate the list):

var checkboxLimit = 4;
var cbl;

function limitCheckboxes(form, event) {

  // Clear checkbox list
  var cblTemp = [];

  // get element that the click came from
  var tgt = event.target || event.srcElement;

  // See if it was a reset
  var reset = tgt.type == 'reset';

  // Count checked checkboxes
  // If click was from reset, reset the checkbox first
  var cb, cbs = form.getElementsByTagName('input');

  for (var i=0, iLen=cbs.length; i<iLen; i++) {
    cb = cbs[i];

    if (cb.type == 'checkbox') {
      cb.checked = reset? cb.defaultChecked : cb.checked;
      if (cb.checked) {
        cblTemp.push(cb.id);
      }
    }
  }

  // If too many, remove the last one that was checked,
  // uncheck it and show a message
  if (cblTemp.length > checkboxLimit && tgt.type == 'checkbox') {
    cblTemp.splice(cblTemp.indexOf(tgt.id), 1);
    tgt.checked = false;
    console.log('removed ' + tgt.id);
  }

  // update cbl
  cbl = cblTemp;

  // Debug
  console.log(cbl);
}

You can also get the checked checkboxes in one go using querySelectorAll:

  var cb, cbs = document.querySelectorAll('input[type="checkbox"]');

and the for loop is simpler:

  for (var i=0, iLen=cbs.length; i<iLen; i++) {
    cb = cbs[i];
    cb.checked = reset? cb.defaultChecked : cb.checked;

    if (cb.checked) {
      cblTemp.push(cb.id);
    }
  }

It would be even better to use a class, so you can have other checkboxes that aren't part of the restricted set.

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