Passing a class as a parameter in Typescript functions

Question

Passing a class as a parameter in Typescript functions

When working with Angular 2 testing utilities, I usually follow this process:

fixture = TestBed.createComponent(EditableValueComponent);

The EditableValueComponent is just a standard component class that I use.

I am curious about the inner workings:

static createComponent<T>(component: Type<T>): ComponentFixture<T>;

This has me thinking, as I want to streamline certain testing procedures:

export class SuperFixture<T>
{  
    fixture: ComponentFixture<T>;
    component: T;

    constructor()
    {         
        this.fixture = TestBed.createComponent(T); // <--- encountered an issue here!
        this.component = this.fixture.componentInstance;   
    }
}

The problem at hand is described as follows:

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

EDIT #1

To resolve the issue, I made the following adjustment:

constructor(component)
{
    this.fixture = TestBed.createComponent<T>(component);

Even after resolving it, the mechanics behind it all still puzzle me.

javascript angular typescript

Answer 1

Answer №1

Make sure to provide the actual constructor function of a class (which creates instances of that class) to the constructor function of MegaFixture. When you use TestBed.createComponent, it internally uses the provided constructor function along with new keyword to instantiate objects of the specified class. The definition of MegaClass could be something like this:

class MegaFixture<T>
{
  fixture: ComponentFixture<T>;
  component: T;

  // passing the constructor function for creating instances of T
  constructor(componentConstructor: new () => T)
  {
    this.fixture = TestBed.createComponent<T>(componentConstructor);
    this.component = this.fixture.componentInstance;
  }
}

Answer 2

Make sure to provide the actual constructor function of a class (which creates instances of that class) to the constructor function of MegaFixture. When you use TestBed.createComponent, it internally uses the provided constructor function along with new keyword to instantiate objects of the specified class. The definition of MegaClass could be something like this:

class MegaFixture<T>
{
  fixture: ComponentFixture<T>;
  component: T;

  // passing the constructor function for creating instances of T
  constructor(componentConstructor: new () => T)
  {
    this.fixture = TestBed.createComponent<T>(componentConstructor);
    this.component = this.fixture.componentInstance;
  }
}

Answer 3

Answer №2

Had to take a quick break for some java while in the middle of crafting that response. ¯\_(ツ)_/¯

The concept you are utilizing is referred to as a Generic within TypeScript. It enables the definition of types dynamically with "type variables" (such as <T>), distinct from function parameters.

Previously, the type variable was being supplied as a function parameter, when the function required an instance of type T. This is what caused the error message.

Your adjustment now functions correctly because you are passing both the type variable and the instance in their appropriate positions during the invocation.

Upon instantiation, the SuperFixture object captures the value of T, which it subsequently transmits to createComponent in its constructor alongside the value of component.

Answer 4

Had to take a quick break for some java while in the middle of crafting that response. ¯\_(ツ)_/¯

The concept you are utilizing is referred to as a Generic within TypeScript. It enables the definition of types dynamically with "type variables" (such as <T>), distinct from function parameters.

Previously, the type variable was being supplied as a function parameter, when the function required an instance of type T. This is what caused the error message.

Your adjustment now functions correctly because you are passing both the type variable and the instance in their appropriate positions during the invocation.

Upon instantiation, the SuperFixture object captures the value of T, which it subsequently transmits to createComponent in its constructor alongside the value of component.

Passing a class as a parameter in Typescript functions

Answer №1

Answer №2

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