Can functions be stored and invoked within a dictionary in TypeScript?

Currently, I'm in the process of refactoring some code and had a question regarding the possibility of declaring and initializing a dictionary that contains factory functions, with each function being associated with an enumerator key. This setup would allow for easy lookup and invocation of the factory functions. Alternatively, I am open to suggestions if there is a more elegant solution that I might be overlooking. I came across this helpful answer which guided me on how to declare and initialize a typed dictionary, but I have concerns about whether I've correctly defined the signature so that the key is a number and the value is a function. To illustrate my query, I have simplified the code into a generic example - though it may seem contrived, it serves its purpose effectively.

// Various types are enumerated as I have multiple type lists that I want to implement as arrays of enumerators
enum ElementType {
    TypeA,
    TypeB,
    TypeC
}

// Here, I aim to define a dictionary where the keys are numbers and the values are functions
var ElementFactory: { [elementType: number]: () => {}; };

// Subsequently, I specify these factory functions to create new objects
ElementFactory[ElementType.TypeA] = () => new ElementOfTypeA();
ElementFactory[ElementType.TypeB] = () => new ElementOfTypeB();
ElementFactory[ElementType.TypeC] = () => new ElementOfTypeC();

// Finally, I desire to call these functions to obtain instances of declared objects as shown in the code block above
var a = ElementFactory[ElementType.TypeA]();
var b = ElementFactory[ElementType.TypeB]();
var c = ElementFactory[ElementType.TypeC]();

Answer №1

Your implementation is solid overall and the concept is sound, but there is room for enhancement:

// I am defining a dictionary where keys are numbers and values are functions
var ElementFactory: { [elementType: number]: () => {}; };

When you see () => {} in a type definition, it signifies "a function that takes no parameters and returns an empty object". To make it more precise, consider specifying the return type. Nonetheless, you will still need to manually specify the returned value types when using these factory functions. Here's how you can do that:

type AnyElementType = ElementOfTypeX | ElementOfTypeY | ElementOfTypeZ;

var ElementFactory: { [elementType: number]: () => AnyElementType; };

...

// this type declaration may not work as expected
var x: ElementOfTypeX = ElementFactory[ElementType.TypeX]();

// whereas these will work fine
var y = <ElementOfTypeY>ElementFactory[ElementType.TypeY]();
var z = ElementFactory[ElementType.TypeZ]() as ElementOfTypeZ;

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