KnockoutJS: computed vs. pureComputed

Question

What's the difference between computed and pureComputed in KnockoutJS?

Can I use pureComputed instead of computed safely?

Answer 1

They are very similar. The difference is that pureComputed has some performance optimizations, and tries to prevent memory leaks, by being smart about who's tracking its changes.

You can safely replace computed with pureComputed in a lot of cases. The function inside the computed should follow this:

1.Evaluating the computed observable should not cause any side effects.

2.The value of the computed observable shouldn’t vary based on the number of evaluations or other “hidden” information. Its value should be based solely on the values of other observables in the application, which for the pure function definition, are considered its parameters.

So as a rule of thumb, any computed observable that just plainly transforms some regular observable properties should be fine as a pureComputed, otherwise stick with computed.

The documentation has decent explanations of when/why you should not use pureComputed observables. Here's a relevant excerpt:

You should not use the pure feature for a computed observable that is meant to perform an action when its dependencies change.

The reason you shouldn’t use a pure computed if the evaluator has important side effects is simply that the evaluator will not run whenever the computed has no active subscribers (and so is sleeping). If it’s important for the evaluator to always run when dependencies change, use a regular computed instead.

Answer 2

I agree with the @Jeroen and I would like to add a short example from J. Munro's book which helped me a lot so this might be helpful for others as well.

First of all, pureComputed observables are quite similar to the computed observables with several performance and memory improvements. The name is borrowed from the Pure function programming term and it means that any function which uses only local variable is potentially pure, whereas any function that uses a non-local variable is potentially impure.

The observables in Knockout.js are treated differently. Thus pureComputed observables are placed in a sleeping mode (Knockout inclines all dependencies and re-evaluates the content when after reading) and computed observables are placed into listening mode (Knockout constantly checks whether the value is up-to-date prior to first access).

Therefore, if you need to execute other code, then better to use a computed observables.

function ViewModel() {
     var self = this;

     self.firstName = ko.observable('Arshile');
     self.lastName = ko.observable('Gorky');
     self.pureComputedExecutions = 0;
     self.computedExecutions = 0;

     self.pureComputedFullName = ko.pureComputed(function() {
         // This is NOT recommended 
         self.pureComputedExecutions++;
         return 'Hello ' + self.firstName() + ' ' + self.lastName();
     });
     self.computedFullName = ko.computed(function() {
         self.computedExecutions++;

         return 'Hello ' + self.firstName() + ' ' + self.lastName();
     });
 };
 var viewModel = new ViewModel();
 ko.applyBindings(viewModel);

 alert('Pure computed executions: ' + viewModel.pureComputedExecutions);
 alert('Computed executions: ' + viewModel.computedExecutions);

When this code is run, two alert messages are displayed that show the number of times the pureComputed and computed functions are called. Since pureComputed is in sleeping mode then the function has never been accessed, and the counter wil display 0. In contrast to this, the computed function is automatically evaluated on data binding, causing the counter to increase the number and display 1.