[BUG] datasets[0].data.shift() causes an error

[nagix]

Expected Behavior

The code below should work.

var ctx = document.getElementById('myChart').getContext('2d');
var chart = new Chart(ctx, {
    type: 'bar',
    data: {
        labels: ['1', '2', '3'],
        datasets: [{
            data: [0, 2, 4]
        },{
            data: [1, 3, 5]
        }]
    }
});

chart.data.datasets.shift();
chart.data.datasets[0].data.shift(); // ---(1)
chart.update();

Current Behavior

The following error occurs at (1) of the code above.

[Error] TypeError: undefined is not an object (evaluating 'dataset._meta')
	getDatasetMeta (Chart.js:4331)
	onDataShift (Chart.js:4886)
	(anonymous function) (Chart.js:4617)
	each (Chart.js:5052)
	value (Chart.js:4615)
	onload (show:65)"

Possible Solution

It seems that an array event listener is called when the data array has been altered by 'push', pop', 'shift', 'splice' or 'unshift'. However, if the dataset array has been altered right before, listeners may try to access meta data that doesn't exist as they are not aware of changes in its parent dataset.

We may need to monitor changes in not only data but also datasets.

Steps to Reproduce

This script has no problem:
https://jsfiddle.net/nagix/vo17cz5y/

This causes the error above:
https://jsfiddle.net/nagix/s2zpf4dq/

Environment

• Chart.js version: 2.6.0
• Browser name and version: Chrome 59.0.3071.109, FireFox 54.0, Safari 10.1.1

[sam-lex]

I never tried shifting/pushing..popping .., instead I always assigned a new array for both labels and datasets.
So I thought that it caused the error because the dataset didn't have enough data after one element was removed (not matching labels.length), but not really heh..

I changed the order, and this worked.. it's definitely a bug
https://jsfiddle.net/s2zpf4dq/2/

[etimberg]

@nagix @simonbrunel @kurkle @benmccann I think I've got a solution, described below, which will work. Thoughts?

1. Move the array listener code out of the dataset controller and into a helper
2. In the main controller, somewhere around here some code is added to remove the old listeners and bind the new listeners to the data.datasets array
3. When changes are made to the dataset array, the meta data is updated accordingly
4. When an item is removed from the dataset array, the existing listeners are removed

[etimberg]

@nagix @kurkle @benmccann looks like this has different behaviour on master: https://jsfiddle.net/03ghtuo6/

[kurkle]

I think this was fixed by #6864

[etimberg]

Sweet, I'll close this and mark it as in V3 🎉