Hierarchy: Obtain the parent of a specific type through type checking

Question

Hierarchy: Obtain the parent of a specific type through type checking

I'm struggling with TypeScript's type checking system, especially when dealing with a composite that contains elements derived from a common base class. How can I create a function that recursively traverses the hierarchy to find and return the first ancestor of a specific type?

abstract class Employee
{
    public Superior: Employee;

    /** THIS IS NOT WORKING */
    public getSuperiorOfType<T extends Employee>( type: typeof T ): T
    {
        if (this instanceof T) return this;
        else if (this.Superior !== undefined) return this.getSuperiorOfType(type);
    }
}

class Manager extends Employee {}
class TeamLead extends Employee {}
class Developer extends Employee {}

let tom = new Manager();
let suzanne = new TeamLead();
let ben = new Developer();

ben.Superior = suzanne;
suzanne.Superior = tom;

let x = ben.getSuperiorOfType(Manager); // x = tom

Any assistance on this matter would be greatly appreciated...

typescript types typechecking

Answer 1

Answer №1

The declaration of 'class type' cannot be made using typeof T. The use of typeof in a type position is restricted to variables only, not types. To achieve this, you should utilize what is known as a constructor signature:
```
public getSuperiorOfType<T extends Employee>(type: { new(...args: any[]): T}): T
```
Attempting to verify if an object is an instanceof a generic type parameter will not work - instanceof T is ineffective because generic type parameters are not present at runtime. However, since you have the type as an actual function parameter, utilizing instanceof type may be successful.

Your code lacks true recursion as each call to getSuperiorOfType is made on the same this object. In order to move up one level in hierarchy, you need to invoke it as

this.Superior.getSuperiorOfType(...)

.

abstract class Employee
{
    public Superior: Employee;

    public getSuperiorOfType<T extends Employee>(type: { new(...args: any[]): T}): T
    {
        if (this instanceof type) return this;
        else if (this.Superior !== undefined) return this.Superior.getSuperiorOfType(type);
    }
}

class Manager extends Employee {}
class TeamLead extends Employee {}
class Developer extends Employee {}

let tom = new Manager();
let suzanne = new TeamLead();
let ben = new Developer();

ben.Superior = suzanne;
suzanne.Superior = tom;


let x = ben.getSuperiorOfType(Manager); 
console.log(x === tom); // true

Answer 2

The declaration of 'class type' cannot be made using typeof T. The use of typeof in a type position is restricted to variables only, not types. To achieve this, you should utilize what is known as a constructor signature:
```
public getSuperiorOfType<T extends Employee>(type: { new(...args: any[]): T}): T
```
Attempting to verify if an object is an instanceof a generic type parameter will not work - instanceof T is ineffective because generic type parameters are not present at runtime. However, since you have the type as an actual function parameter, utilizing instanceof type may be successful.

Your code lacks true recursion as each call to getSuperiorOfType is made on the same this object. In order to move up one level in hierarchy, you need to invoke it as

this.Superior.getSuperiorOfType(...)

.

abstract class Employee
{
    public Superior: Employee;

    public getSuperiorOfType<T extends Employee>(type: { new(...args: any[]): T}): T
    {
        if (this instanceof type) return this;
        else if (this.Superior !== undefined) return this.Superior.getSuperiorOfType(type);
    }
}

class Manager extends Employee {}
class TeamLead extends Employee {}
class Developer extends Employee {}

let tom = new Manager();
let suzanne = new TeamLead();
let ben = new Developer();

ben.Superior = suzanne;
suzanne.Superior = tom;


let x = ben.getSuperiorOfType(Manager); 
console.log(x === tom); // true

Hierarchy: Obtain the parent of a specific type through type checking

Answer №1

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