Why can't canActivate just do it? Why would I need to have logic that runs only once at the expense of having to provide both canActivate and canLoad to ensure I both check the condition every time and that the code is not unnecessarily loaded?

 The difference between canLoad and canActivate guards in Angular lies in their behavior and when they are triggered during routing. Here's a detailed explanation:

1. canLoad Guard:

Purpose: It is used to prevent the loading of a module or route before it is loaded.

When it's triggered: It runs before the route module (or lazy-loaded module) is loaded. This is especially useful when you want to prevent unnecessary loading of large modules or routes.

Why it only runs once: Once the route module is loaded, the logic for that route is effectively "done". The module is now available in the application and won't be checked again unless the user navigates away and back to the route (unless the module is unloaded and reloaded via some custom setup).

Key advantage: It helps reduce the overhead of loading a route if the condition for accessing it isn't met. For example, if a user doesn't have the proper permissions, the route won't even be loaded.

2. canActivate Guard:

Purpose: It is used to check conditions before navigating to a route, and it runs every time the route is navigated to.

When it's triggered: It runs when the router tries to navigate to the route and before activating the route’s components. If the guard returns true, the navigation continues, otherwise, it is blocked.

Why it runs every time: canActivate checks conditions every time the user navigates to the route. This is useful for ensuring that a user has the necessary permissions or that the conditions are still valid (e.g., rechecking authentication or permissions).

Key advantage: It allows for dynamic checks before navigation, ensuring that a user can access the route only under specific conditions (like an updated authentication status or dynamic role permissions).

Why both canLoad and canActivate are useful:

Both guards serve different purposes and provide complementary functionality.

canLoad prevents loading the module entirely if the condition isn’t met, which is important to save resources, especially when the route is lazily loaded. If the condition for loading the module is not met, the guard can block the module loading upfront. Once the module is loaded, canLoad won’t be called again.

canActivate runs every time the user navigates to the route, which makes it necessary to use it if you want to always check permissions or any conditions (e.g., authentication status) each time the user tries to access the route. This ensures that even if the route is already loaded, the guard will prevent access if the conditions change.

Why canActivate can’t just replace canLoad:

Performance considerations:

canLoad runs before the module is loaded, which means if the condition fails, the module will never be loaded. If you only used canActivate, the module would still load, potentially wasting resources by downloading large chunks of code or making unnecessary network requests (e.g., lazy-loaded modules).

With canLoad, you avoid these unnecessary costs by not loading the module in the first place, which can be a major performance optimization for larger applications.

Different Responsibilities:

canLoad is about preventing the loading of modules before they are even needed, whereas canActivate deals with checking conditions every time the user tries to navigate to a route.

canLoad ensures that large, unnecessary modules don’t get loaded, while canActivate ensures that the user has access to the route before they can interact with it.

Conclusion:

You need both canLoad and canActivate for different scenarios:

Use canLoad to prevent loading the module when certain conditions aren’t met (e.g., access control, permission checks). This helps optimize performance by not loading unnecessary code.

Use canActivate to check conditions every time the route is activated, ensuring that the user can only access the route if the necessary conditions are met, such as authentication or authorization.

This separation allows you to maintain efficient code splitting (lazy loading) while ensuring that routes are protected based on dynamic conditions (like user state or permissions).