Angular Interview Practice

What distinguishes pure pipes from impure pipes in Angular?

1. Pure pipes retain state, impure do not

2. Pure pipes are synchronous, impure are asynchronous

3. Pure pipes do not re-evaluate on every change detection cycle, impure do

4. Pure pipes can only accept one parameter, impure can accept multiple

The correct answer is: Pure pipes do not re-evaluate on every change detection cycle, impure do

Pure pipes in Angular are designed to enhance performance by minimizing unnecessary recalculations. They are evaluated only when the input parameters change, meaning that Angular does not reevaluate them on every change detection cycle if the inputs remain the same. This characteristic leads to improved efficiency, especially in applications where data changes frequently but the pipe's input remains stable. In contrast, impure pipes are evaluated with every change detection cycle, regardless of whether their input values have changed. This can lead to performance issues if the pipe is used extensively in templates, as it forces recalculations which can slow down the application. The other concepts addressed in the options do not accurately capture the differences between pure and impure pipes. For example, the idea of retaining state does not distinctly characterize pure from impure pipes; both can potentially have state, but pure pipes focus on input changes. Similarly, both types of pipes can work synchronously or asynchronously depending on their implementation, and both can accept multiple parameters—although they are usually constrained by their intended use cases. Thus, the fundamental distinction is based on when the pipes re-evaluate in relation to change detection cycles, which is clearly explained in the correct choice.