Using Typescript with Angular: passing a generic Type to a function

Question

Using Typescript with Angular: passing a generic Type to a function

When using Typescript for Angular 5 unit tests, I have created a function that can query the DOM and return an instance of the MyComponent class if it exists:

function getMyComponent(hostFixture: ComponentFixture<any>): MyComponent {
  const debugElement = hostFixture.debugElement.query(By.directive(MyComponent));

  return (debugElement && debugElement.componentInstance) || null;
}

This function is working perfectly. Now, I want to make it more versatile so that it can query any Type as needed. I thought this could be achieved with generic types. The updated function call would look like this:

const instance: MyComponent = getDirective<MyComponent>(fixture);

However, I am facing issues getting this new version of the function to work correctly. Here is the problematic part:

function getDirective<T>(hostFixture: ComponentFixture<any>): T {
  const debugElement = hostFixture.debugElement.query(By.directive(T));

  return (debugElement && debugElement.componentInstance) || null;
}

The error message that appears says:

TS2693: 'T' only refers to a type, but is being used as a value here

I'm struggling to figure out the correct syntax in this scenario. The function I am using from Angular -- By.directive() -- expects a parameter of type Type<any>, which is documented here

angular typescript generics

Answer 1

Answer №1

Angular's Type<T> can be a bit misleading in its naming. Rather than representing a general type, it actually refers to something that is constructable - a value that can be called with new.

When using By.directive, keep in mind that it takes a value, not just a type. This value should be something that can be instantiated, like a class or a function.

While you may feel constrained by the existing function, you do have the flexibility to write your own version if needed.

Here is an alternative way of approaching it:

export function getDirective<T>(
  hostFixture: ComponentFixture<any>, 
  ComponentConstructor: new (...args: unknown[]) => T
): T {
  const debugElement = hostFixture.debugElement.query(By.directive(ComponentConstructor));

  return debugElement && debugElement.componentInstance || undefined;
}

You would then call this function as follows:

const instance = getDirective(fixture, MyComponent);

Additional Thoughts:

It's important to note that TypeScript types are primarily used at design time and do not impact runtime behavior. The name Type<T> can be confusing for those new to Angular and TypeScript, as it may lead to misconceptions about classes and types.

Frameworks like Angular tend to blur the lines between classes and types, but it's essential to recognize that they are distinct concepts. While frameworks like Aurelia may also encourage this conflation, they offer more clarity by allowing dependencies to be specified differently.

Despite some confusion, it's worth acknowledging the efforts made by the Angular team in developing their own language, @Script. Although ultimately they opted for TypeScript, traces of @Script's influence can still be seen within Angular today.

Answer 2

Angular's Type<T> can be a bit misleading in its naming. Rather than representing a general type, it actually refers to something that is constructable - a value that can be called with new.

When using By.directive, keep in mind that it takes a value, not just a type. This value should be something that can be instantiated, like a class or a function.

While you may feel constrained by the existing function, you do have the flexibility to write your own version if needed.

Here is an alternative way of approaching it:

export function getDirective<T>(
  hostFixture: ComponentFixture<any>, 
  ComponentConstructor: new (...args: unknown[]) => T
): T {
  const debugElement = hostFixture.debugElement.query(By.directive(ComponentConstructor));

  return debugElement && debugElement.componentInstance || undefined;
}

You would then call this function as follows:

const instance = getDirective(fixture, MyComponent);

Additional Thoughts:

It's important to note that TypeScript types are primarily used at design time and do not impact runtime behavior. The name Type<T> can be confusing for those new to Angular and TypeScript, as it may lead to misconceptions about classes and types.

Frameworks like Angular tend to blur the lines between classes and types, but it's essential to recognize that they are distinct concepts. While frameworks like Aurelia may also encourage this conflation, they offer more clarity by allowing dependencies to be specified differently.

Despite some confusion, it's worth acknowledging the efforts made by the Angular team in developing their own language, @Script. Although ultimately they opted for TypeScript, traces of @Script's influence can still be seen within Angular today.

Using Typescript with Angular: passing a generic Type to a function

Answer №1

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