Tips for selecting specific types from a list using generic types in TypeScript

Question

Tips for selecting specific types from a list using generic types in TypeScript

Can anyone assist me in creating a function that retrieves all instances of a specified type from a list of candidates, each of which is derived from a shared parent class?

For example, I attempted the following code:


        class A {
            protected children: A[] = [];
        
            getChildrenOfType<T extends A>(): T[] {
                let result: T[] = [];
                for (let child of this.children) {
                    if (child instanceof T)
                        result.push(<T>child);
                }
            
                return result;
            }
        }

using classes like:


        class B: extends A {}
        class C: extends B {}
        class D: extends A {}

However, the code does not compile. The error message "'T' only refers to a type but is being used as a value here" appears when using child instanceof T. This issue seems to be related to the generic type. What is the correct approach to solve this problem? Do I need additional steps to filter objects based on generic types?

javascript typescript

Answer 1

Answer №1

Feel free to utilize the following code snippet.

class A {
    protected children: A[] = [];

    getChildrenOfType<T extends A>(t: new (...args: any[]) => T): T[] {
        let result: T[] = [];
        for (let child of this.children) {
            if (child instanceof t)
                result.push(<T>child);
        }

        return result;
    }
}

Make sure that the right side of the instanceof is a constructor function, such as new() => MyClass. You can pass that parameter to the method.

The getChildrenOfType function can be implemented in the following way:

var a = new A();
// Add some children to a.children
a.getChildrenOfType(B)

Test it out on the playground.

Answer 2

Feel free to utilize the following code snippet.

class A {
    protected children: A[] = [];

    getChildrenOfType<T extends A>(t: new (...args: any[]) => T): T[] {
        let result: T[] = [];
        for (let child of this.children) {
            if (child instanceof t)
                result.push(<T>child);
        }

        return result;
    }
}

Make sure that the right side of the instanceof is a constructor function, such as new() => MyClass. You can pass that parameter to the method.

The getChildrenOfType function can be implemented in the following way:

var a = new A();
// Add some children to a.children
a.getChildrenOfType(B)

Test it out on the playground.

Tips for selecting specific types from a list using generic types in TypeScript

Answer №1

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