What is an impure pipe in Angular?

An impure pipe in Angular is a type of pipe that is not pure, meaning it does not always return the same output for a given input. In Angular, pipes are used to transform input data into a desired output format, and impure pipes can be used when the output needs to be recalculated every time change detection runs, rather than once the input changes.

Impure pipes are denoted by setting the pure property of the @Pipe decorator to false. This tells Angular that the pipe is impure and needs to be re-evaluated on every change detection cycle.

Impure pipes are useful in situations where the input data is dynamic and can change frequently, and the output of the pipe needs to reflect those changes immediately. For example, if you have a list of items and want to filter them based on some criteria, using an impure pipe ensures that the filtered list is always up to date without needing to manually trigger the filtering process.

When using impure pipes, it’s important to be mindful of performance implications, as re-evaluating the pipe on every change detection cycle can potentially impact the overall application performance. It’s recommended to use impure pipes sparingly and only when necessary.

In conclusion, impure pipes in Angular are a way to handle dynamic input data and ensure that the output reflects the changes immediately. By setting the pure property to false, you indicate to Angular that the pipe is impure and needs to be re-evaluated on every change detection cycle.