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Wishing For A Lazier Underscore

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Wishing For A Lazier Underscore

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Sometimes the day of a developer gets awfully complicated. One of my less favorite moments are those when you have used a library for most of your project and then you realize you need things from it it was not designed to do. I was in this particular position last week, trying to stretch UnderscoreJS in ways it just wasn’t designed to bend.

Don’t get me wrong, I love UnderscoreJS, it brings the sweetness of C#’s LINQ-style chained queries to JavaScript arrays. If it was a person, I would totally hug it. But it made me smash my head against the wall a couple times when I realized that it lacks lazy evaluation of chained queries.

Why would I need that, you ask? Because sometimes I have to accomplish the darnest requirements. The original culprit was Kendo UI, an awesome UI-widget library by the guys at Telerik. The issue is that I had to completely override its built-in filtering mechanism since I needed it to be able to filter a data source in a master-detail fashion. For example, I have an array of customers, each customer has a sub-array of products it has bought. The grid displays a master row per-customer and detail rows for the products it has bought. Even so, the user should be able to filter by product, so if any customer had bought a particular product, it should be listed in the grid.

As long as I (and Google) know, this cannot be done out-of-the-box with Kendo UI unless you do it manually on server-side, since it can filter over all of the object properties, but not sub-arrays.

After some work, I managed to override the default filters and the original filter method in the grid’s data source. However, I needed a fast way to filter the data source like the original filtering mechanism did, so I recurred to UnderscoreJS. Performance was not that good; after wrapping the data source in an UnderscoreJS chain, I had to apply the filter method once per filter applied on the grid. The more filters, the worst performance got.

I eventually figured out that every time I was calling the filter function, it was iterating once over the entire data source. Of course performance was bad. Really dumb of me, since I was used to how LINQ worked; it allowed me to build a query in a fluent fashion and then execute it over the data source so it was iterated over just once. This process is called lazy evaluation of a query over a data source. UnderscoreJS does NOT feature lazy evaluation.

var filteredCustomers = 
        .filter(function (customer) { // Iterates the array once...
            return customer.firstName === 'Luis';
        .filter(function (customer) { // And then again...
            return customer.lastName === 'Aguilar';
        .filter(function (customer) { // And again...
            return _(customer.products).any(function (product) { // LOL don't even ask...
                return product.name === 'Time Machine X9000-S'

I think the only thing left to do is to use another library to support my custom filtering mechanism. Sigh. I’ll let you know of my findings…

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