Demystifying Angular Pipes: Understanding Pure and Impure Pipes

 In Angular, pipes are used to transform data in templates. They are a way to format, filter, or manipulate data before displaying it to the user. Angular pipes can be categorized into two types: pure and impure.

1. Pure Pipes:

   1. Pure pipes are the default type of pipe in Angular.
   2. They are called pure because they are stateless and do not have any internal state that can change over time.
   3. Pure pipes are highly optimized and Angular executes them only when the input values to the pipe change.
   4. If the input values remain the same, Angular will use the previously cached result, which improves performance.
   5. Examples of pure pipes include UpperCasePipe, LowerCasePipe, CurrencyPipe, and DatePipe.

2. Impure Pipes:

   1. Impure pipes, as the name suggests, are the opposite of pure pipes.
   2. They have internal state or rely on external factors that can change during the component lifecycle.
   3. Impure pipes are called more frequently as Angular does not cache the result and executes them on every change detection cycle.
   4. Using impure pipes extensively can degrade performance, as they are called frequently and can impact the overall application speed.
   5. To explicitly mark a pipe as impure, you can set the pure property of the pipe's metadata to false.
   6. It's generally recommended to avoid using impure pipes unless necessary and instead consider using other techniques like subscribing to observables or handling the transformations in the component itself.

To summarize, pure pipes are the default type in Angular and provide better performance by caching results and executing only when the input values change. Impure pipes, on the other hand, have internal state or rely on external factors, causing them to be called more frequently and potentially impacting performance. It's advisable to use pure pipes whenever possible and use impure pipes sparingly, considering their potential performance implications.