Employing TypeScript decorators to enforce a required parameter in functions

Working on a typescript project, I encountered over 2000 events(methods) that are triggered during user operations. To ensure performance metrics for all events related to the completion of an operation, I decided to make 'executionTime' a mandatory parameter with a method decorator. Here's a snippet of my code:

class AllEvents {

    @performanceDecorator(true)
    public static firstEvent(param1: string, param2: string, executionTime: number): void {
        //some tasks...
    }

    @performanceDecorator(true)
    public static secondEvent(param1: string, param2: string): void {
        //some tasks...
    }

    @performanceDecorator(true)
    public static thirdEvent(param1: string, param2: string, executionTime: number): void {
        //some tasks...
    }
}

function performanceDecorator(value: boolean) {
    //..................
}

Being new to typescript, I'm struggling to determine how to implement the logic inside the performanceDecorator function. My goal is to throw errors if a method does not include the 'executionTime' parameter.

Answer №1

Building upon my previous response

In your specific scenario where the number of parameters is not fixed, it becomes challenging to identify which parameter represents the execution time.

To address this issue, consider creating an options class with the expected parameters as properties:

class EventOptions {
    public params: string[];
    public executionTime?: number;
}

class AllEvents {

    @performanceDecorator
    public static firstEvent(eventOptions: EventOptions): void {
        //perform tasks...
        console.log(eventOptions.params, eventOptions.executionTime);
    }

    @performanceDecorator
    public static secondEvent(eventOptions: EventOptions): void {
        //perform tasks...
    }

    @performanceDecorator
    public static thirdEvent(eventOptions: EventOptions): void {
        //perform tasks...
    }
}

In this example, 'executionTime' is set as an optional parameter for flexibility. However, you can make it a required field if needed, eliminating the need for a decorator to validate its presence as TypeScript will handle that automatically.


Next, let's delve into the decorator functionality:

A function decorator includes three parameters:

  • target: Referring to the invoking class, in this case AllEvents
  • propertyKey: Representing the method name as a string
  • descriptor: Containing metadata about the function

The decorator implementation could be written as follows:

function performanceDecorator(
    target: Object,
    propertyKey: string,
    descriptor: PropertyDescriptor) {
        const originalMethod = descriptor.value;

        descriptor.value = (...args) => {
            if (!args[0].executionTime) {
                throw Error("Execution Time is missing!");
            }
            return originalMethod.apply(this, args);
        }
        return descriptor;
}

In this decorator, we store the original method and then utilize a condition to verify the presence of the 'executionTime' parameter. If absent, an error is thrown; otherwise, the original method is executed.

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