Angular Routes And Optional Params

December 27, 2017

NOTE: The method mentioned below is only needed in case your parameters are likely to change after routing to a component. Otherwise, just use router.snapshot.paramMap.

I recently encountered a situation while building an Angular app, where I needed to handle route params and optional params at the same time. Obviously, I used GET params as the optional parameters, as is the standard on the web. However, it turns out that the Angular Router provides separate Observables for handling route params, and query params.

Route Params:

this.router.params.subscribe(params => {
  // do something with route params
});

Query Params:

this.router.queryParams.subscribe(params => {
  // do something with query params
});

In a scenario where you have the same component across pages, the component is not re-initialized. Which means that you might not receive a callback if you navigate to the same page, or a different page with different URL parameters, pointing to the same component. The router will not send you a callback if you navigate to the same URL either. Which means, you can't wait for both Observables to resolve, because there is no guarantee of both resolving.

The problem was that I needed both parameters to resolve before I could start rendering the component.

The Solution

After googling and reading through various blog posts, I was unable to find a good solution to this problem. Finally, I decided to RTFM, which in hindsight, I should have done sooner. In a huge page documenting the router behavior, there are a few lines that mention this scheme. The Angular team has made use of Matrix URL notation which involves optional arguments appearing in the URL separated by semicolons.

ParamMaps have been introduced in Angular 4.0, and they support Matrix Params. Thus, the solution becomes simple:

this.router.paramMap.subscribe(params => {
  // do something with optional and required
});

Not just that, the ParamMap class provides nice getter methods to make it easier to access single, or multiple values. At the time of writing this post, Router.paramMap does NOT support GET params.

Hope this helps someone looking for optional arguments in Angular routes.