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How can I set a a state variable from two UI elements?

So, apologies if this is too much of an open and beginner-like question.
I am trying to build a single page app, in which one can control a variable in two ways:
by dragging the corresponding datapoint on a D3 scatterplot chart
by setting its value with a slider
I would like the chart to update when the slider gets moved, and the slider to move when the chart is updated by dragging the data on the chart.
I have thought of handling this through:
a spaghetti tangle of events
a proxy object mediating the changes
However I'm asking myself (and the illustrious SO community) is there a better way to handle this?

The keyword for solving this is 'data binding'. There are several frameworks out there, which are able to solve this. That means, that you have a javascript model, which holds the value, you want to work with and something like a proxy, which handles changes and stuff and applies the changes to the view or respectively to the mode..
In case you might need this more often, which usually applies to single page apps, I would recommend you to use some framework, which does the work for you instead of reinventing the wheel.
If you want to stay with client based Javascript, you might take a look into Angular.js (never used it, but maaaany people do and really do like it) or Knockout.js (I work with it, but there are no more further developments).
Within the last years and now many peoply switched to Node.js and Angular2 (a further development of Angular.js) or React.js or Vue.js. You might also take a look into it, but going into detail would blow up the answer. Moreover I only know some basics right now and other people can do better and already did better.

Reduce watches created by ui-grid or alternative grid solutions

Apologies if this is a repeated question about ui-grid but I could do with being clear on the options for my particular use case. Other similar questions don't conclusively answer my problem.
We are attempting to use ui-grid in a fairly unusual web application where the number of tables on the page is effectively limitless. They get added the more activity the user does. Not ideal but that is what is required.
When using ui-grid, even with a fairly small simple table, it is adding around 1000 watches for each table. And the digest cycle increases significantly with each table being added too. This leads to poor performance when the number of tables increases, as you might expect.
My question is then, is there anything that can be done with ui-grid to reduce these number of watches? i.e. without delving in to the source code to make changes. We are passing our data in to the ui-grid directive using one-time binding notation, but that makes no difference. From looking at other similar questions, it would seem to me that there is nothing else that can be done, which might well imply that ui-grid is not suitable for us.
Alternatively, is there a way within AngularJs to prevent two way binding within a directive? I'm not aware of such a method, but perhaps someone knows a trick to make this happen?!
Finally, can anyone suggest an alternative grid that would be a better fit for our use case? We are trialling ui-grid due to the functionality it provides, such as column selection, cell formatting, ordering, filtering and so on. Are there better performing AngularJS or other JS grids out there that would cover this functionality? Preferably free to use? Others we've considered checking out are Kendo UI, jqxGrid, ag-grid (though perhaps this needs to be paid for).
Any help is massively appreciated.

My company has run into similar issues though not with ui-grid specifically. You've probably already considered this but the fix for us was horizontal and vertical paging. I don't know much about your use case, but we realized that horizontal and vertical scrolling with a huge grid had the same issues you are running into, but it wasn't really any more functional for the user to use a scroll bar than to use page controls to move between blocks of the grid. At most 500 cells would fit on a single screen at a time and it was very easy to lose your place trying to use a scroll bar.
By doing this we're able to use 1 way bindings (probably with bind-once but I would have to dig into the code to be sure). Even with two way binding the same technique should work for you too if it works with your use case.

if you us ag-Grid, there will be no watches. ag-Grid doesn't use Angular at it's core, so doesn't have watches internally.
ag-Grid has two versions, free and enterprise. if you don't need the enterprise features, then use the free versions.

Displaying sub-data rows inside a row in SlickGrid Table

I've started using SlickGrid and have to implement a kind of functionality where I need to show sub-row inside a parent row of SlickGrid.
Here's the scenario
Here's Asia, Europe is having sub categories in Services Column and these service columns are expandable/collapsible
I'm trying to customize this for first column.
Is any other link/source available to refer to accomplish the feature.

Are you aware of the grouping/aggregates example ?
http://mleibman.github.io/SlickGrid/examples/example-grouping
This offers very similar behaviour, but the exact 'cell-merging' behaviour you show is not currently available in SlickGrid. Slickgrid tends to use tree structures rather than a flat structure. That said, it may not be too hard to hack up, but it seems everyone wants a different visual layout and it becomes very hard to support them all.
If you don't need editing or scrolling, I'd suggest this would work best as an HTML table, or even a PDF.

Dojo Datagrid Sorting Mask

I'm currently working with a dojo datagrid that works perfectly well except for one small problem. Due to a large number of fields that must be accounted for, clicking on a column to sort the contents of the grid by the value within that column can be rather time consuming (several seconds). Is there any way to display to my users that the content of the grid is "loading"/"sorting"/etc during this downtime? I've looked around the web but only found loading masks and no sorting masks. As far as I can tell there are no built-in methods for detecting sorting or changes in sorting.
Thanks in advance.

This Post handles the onHeaderCellClick event which would catch the onClick in the header.
Declarative dojox.grid.datagrid's header has onclick event?
Theres a fiddle in the Post, which disrcibes it very well.
You can use it to make a loadingimage visible and hide it again, when sortig ends.
Regards, Miriam

Javascript widget inspired by iPhone UITableView?

Cocoa Touch's UITableView allows a user to scroll through large numbers of data rows with good performance because it recycles table rows. Rather than create a GUI element for every single data row, a limited number of table rows is created, and simply updated with the relevant data as the user scrolls, giving the illusion of navigating up and down a very large number of table rows.
Has anyone seen this done in javascript? Is there a plugin available anywhere that will do this for me?

infinity.js works well. It will dynamically load 'pages' behind the scenes giving you the appearance that the list has been fully loaded.
More information can be found on their Github page - https://github.com/airbnb/infinity
Additionally, I've forked the project updating it to work with Zepto. I also set it up to use any scrollable div (set up with overflow: scoll) with the class 'scrollable' - https://github.com/elliotcw/infinity
I should add that I made these changes as this is great for large lists on mobile devices, which slow down when you have to many complex elements on the page.

I was looking for this as well, and infinityjs [1] doesn't seem to quite mimic the same interface as UITableView. And it was a problem for my scenario that infinityjs required that the element containing the list items already be added to the DOM.
MegaList [2] came closest to what I wanted. Andrew (author) has done a great job of designing it for mobile first, with touch support etc. One caveat for me was that it appears to assume a strict selection list model and does a little bit more than I'd like a list component to do (e.g. binding to resize events and trying to handle that automatically).
So I started writing a barebones list component, also modeled after the iOS UITableView. It's a work in progress and I'm putting in just what I need. Sources are here https://github.com/shyam-habarakada/js-virtual-list-view. I'm putting in just what I need as I go, and contributors are needed :-)
[1] http://airbnb.github.io/infinity/
[3] https://github.com/triceam/MegaList

Actually the algorithm is not difficult at all. If you know javascript you should be able to write this. The algorithm only needs the height of a cell and the height of the table.
I only know these two:
Apple's Dashcode javascript Framework has an implementation of a Table. You could take a look and see if that is what you need.
Or Cappuccino Framework which is basically Objective-J but behind the scenes is Javascript.

Clusterize.js does exactly that.
It's small and works with any tag (table, lists, divs)