Author: Andreas Plahn

Testing Components with children – Testing Angular

Testing Components with children

  • Rendering a Component with or without its children
  • Checking that the parent and its children are wired up correctly
  • Replacing child Components with fakes
  • Using the ng-mocks library to fake dependencies

Source: Testing Components with children – Testing Angular

“Expression has changed after it was checked” – error in Angular

The “Expression has changed after it was checked” error in Angular indicates that a component property was updated during Angular’s change detection cycle, causing a mismatch between the DOM and the component’s data. To fix this, you can delay the update by using setTimeout, Promise.then, or an asynchronous observable, or update the property in a lifecycle hook before the DOM update. 

Here’s a more detailed explanation and common solutions:

Understanding the Error
  • Angular’s change detection mechanism checks for changes in component properties and updates the DOM accordingly. 
  • The error occurs when a change is made to a component property during this detection cycle, before the DOM has a chance to reflect the change. 
  • This can happen with asynchronous operations like HTTP requests, timers, or event listeners that update data within the change detection cycle. 
Solutions
  1. Delay the Update with setTimeout, Promise.then, or Asynchronous Observable:
    • Wrap the code that modifies the component property in a setTimeout, Promise.then, or an asynchronous observable (like RxJS).
    • This forces the update to happen after the current change detection cycle is complete, preventing the error. 
TypeScript
    setTimeout(() => {      this.myProperty = newValue;    }, 0);
TypeScript
    Promise.resolve().then(() => {      this.myProperty = newValue;    });
TypeScript
    fromEvent(document, 'click').subscribe(() => {      this.myProperty = newValue;    });
  1. 1. Update the Property in a Lifecycle Hook:
    • Move the code that modifies the component property to a lifecycle hook like ngOnInit, ngAfterContentInit, or ngAfterViewInit. 
    • These hooks are called after Angular has completed its initial change detection and DOM updates, so the change is less likely to interfere with the cycle. 
  2. 2. Manually Trigger Change Detection (Use with Caution):
    • Inject ChangeDetectorRef into your component and use detectChanges() to manually trigger change detection. 
    • This can be useful in specific scenarios, but it’s generally best to avoid manually triggering change detection and use asynchronous solutions or lifecycle hooks whenever possible. 
  3. 3. Refactor your code:
    • Identify the component, the binding, and the expression that are causing the issue. 
    • Determine which property is changing the expression’s result between the regular detectChanges and the checkNoChanges checks. 
    • Refactor your code to prevent changes to the property during the change detection cycle. 
    • For example, move the update to a lifecycle hook or use asynchronous operations. 
Debugging
  • The error message and stack trace can help you pinpoint the exact location where the change is happening.
  • Use the Chrome DevTools debugger to set breakpoints and inspect the values of variables during the change detection cycle.
  • Angular University has a blog post on debugging “Expression Changed” errors that can be helpful. 
Important Considerations
  • Avoid updating component properties directly within template expressions or event handlers. 
  • Use the correct lifecycle hooks for your use case to ensure that updates occur at the appropriate time. 
  • If you need to update the DOM or perform other actions that might interfere with change detection, consider using asynchronous operations or manually triggering change detection. 

Angular Testing Library | Testing Library

The problem​

You want to write maintainable tests for your Angular components. As a part of this goal, you want your tests to avoid including implementation details of your components and rather focus on making your tests give you the confidence for which they are intended. As part of this, you want your testbase to be maintainable in the long run so refactors of your components (changes to implementation but not functionality) don’t break your tests and slow you and your team down.

This solution​

The Angular Testing Library is a very lightweight solution for testing Angular components. It provides light utility functions on top of DOM Testing Library in a way that encourages better testing practices. Its primary guiding principle is:

The more your tests resemble the way your software is used, the more confidence they can give you.

So rather than dealing with instances of rendered Angular components, your tests will work with actual DOM nodes. The utilities this library provides facilitate querying the DOM in the same way the user would. Finding form elements by their label text (just like a user would), finding links and buttons from their text (like a user would). It also exposes a recommended way to find elements by a data-testid as an “escape hatch” for elements where the text content and label do not make sense or is not practical.

This library encourages your applications to be more accessible and allows you to get your tests closer to using your components the way a user will, which allows your tests to give you more confidence that your application will work when a real user uses it.

Source: Angular Testing Library | Testing Library

API rate limiter in .NET Core 8

.NET Core 8 have built-in API rate limiter functionality.
Can be used in an API project like this in program.cs:

app.UseRateLimiter();

//(PUT BELOW CODE AFTER LINE: var app = builder.Build();)
//Setup API request rate limiter
//If a rate limit is exceeded in ASP.NET Core's Rate Limiting Middleware 
//HTTP response will have the status code: 429 Too Many Requests
builder.Services.AddRateLimiter(options =>
{
    options.AddFixedWindowLimiter(
        "default",
        limiterOptions =>
        {
            limiterOptions.PermitLimit = 10; // Allow 10 requests
            limiterOptions.Window = TimeSpan.FromSeconds(1); // Per second
            limiterOptions.QueueProcessingOrder = QueueProcessingOrder.OldestFirst;
            limiterOptions.QueueLimit = 2; // Allow 2 queued requests
        }
    );
});

 

Angular profiler for performance tuning | Angular Newsletter

When facing a performance issue, the Profiler is perfect for identifying the problem’s root cause. For instance, it could highlight that one component is much slower than the others and gets refreshed for no good reason, indicating that a different change detection strategy is needed.

Source: Angular profiler for performance tuning | Angular Newsletter

Angular automated migration from ngIf and ngFor directives markup to control blocks

Directives such as *ngIf and *ngFor will soon get deprecated to favor the new control flow blocks.

E.g. old directives:

<ng-container *ngIf="isLoggedIn">...</ng-container>

Can now be replaced with the new syntax:

@if (isLoggedIn) {
   ...
}

An automated migration CLI command is available

ShellMenuNew – Disable/enable ‘New’ menu items in Windows Explorer

ShellMenuNew is a small utility that displays the list of all menu items in the ‘New’ submenu of Windows Explorer. It allows you to easily disable unwanted menu items, so this ‘New’ submenu will display only the items that you need.

Source: ShellMenuNew – Disable/enable ‘New’ menu items in Windows Explorer