n a component which uses Default change detection strategy, when change detection happens, if the pipe is impure, then the transform method will be called. If the pipe is pure, whether there are any changes in input parameters in the transform method from the last call, then transform method will be called.When pipe is pure, transform() method is invoked only when its input arguments change. Pipes are pure by default. If the pipe has internal state (that is, the result depends on the state other than its arguments), set pure to false. In this case, the pipe is invoked on each change detection cycle, even if the arguments have not changed.A pure pipe is only called when Angular detects a change in the value or the parameters passed to a pipe.An impure pipe is called for every change detection cycle no matter whether the value or parameter(s) changes.This is relevant for changes that are not detected by Angularwhen you pass an array or object that got the content changed (but is still the same instance)when the pipe injects a service to get access to other values, Angular doesn't recognize if they have changed.