Parameter Decorators

A Parameter Decorator is declared just before a parameter declaration. The parameter decorator is applied to the function for a class constructor or method declaration. A parameter decorator cannot be used in a declaration file, an overload, or in any other ambient context (such as in a declare class).

The expression for the parameter decorator will be called as a function at runtime, with the following three arguments:

Either the constructor function of the class for a static member, or the prototype of the class for an instance member.
The name of the member.
The ordinal index of the parameter in the function’s parameter list.

NOTE A parameter decorator can only be used to observe that a parameter has been declared on a method.

The return value of the parameter decorator is ignored.

The following is an example of a parameter decorator (@required) applied to parameter of a member of the Greeter class:

class Greeter {
  greeting: string;
 
  constructor(message: string) {
    this.greeting = message;
  }
 
  @validate
  greet(@required name: string) {
    return "Hello " + name + ", " + this.greeting;
  }
}

We can then define the @required and @validate decorators using the following function declarations:

import "reflect-metadata";
 
const requiredMetadataKey = Symbol("required");
 
function required(target: Object, propertyKey: string | symbol, parameterIndex: number) {
  let existingRequiredParameters: number[] = Reflect.getOwnMetadata(requiredMetadataKey, target, propertyKey) || [];
  existingRequiredParameters.push(parameterIndex);
  Reflect.defineMetadata(requiredMetadataKey, existingRequiredParameters, target, propertyKey);
}
 
function validate(target: any, propertyName: string, descriptor: TypedPropertyDescriptor<Function>) {
  let method = descriptor.value;
  descriptor.value = function () {
    let requiredParameters: number[] = Reflect.getOwnMetadata(requiredMetadataKey, target, propertyName);
    if (requiredParameters) {
      for (let parameterIndex of requiredParameters) {
        if (parameterIndex >= arguments.length || arguments[parameterIndex] === undefined) {
          throw new Error("Missing required argument.");
        }
      }
    }
 
    return method.apply(this, arguments);
  }
}

The @required decorator adds a metadata entry that marks the parameter as required. The @validate decorator then wraps the existing greet method in a function that validates the arguments before invoking the original method.

NOTE This example requires the reflect-metadata library. See Metadata for more information about the reflect-metadata library.

Links:

https://www.typescriptlang.org/docs/handbook/decorators.html#parameter-decorators

doc_TypeScript

2025-01-10 15:47:30

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