[Method decorator] Example how to create method decorator #typescript

method-decorator-example.ts

// Define the decorator
// if the method parameter is not passed 
// then the decorator will return the result of someOtherFunction()
// as value for the parameter
export const decoratorFunction = (index: number) => (
  target: any,
  key: string,
  propDesc: PropertyDescriptor
) => {
  let originalFunction: Function = propDesc.value;
  propDesc.value = function () {
    let argValue = arguments[index];
    let newArgs = [];

    for (let i = 0; i < arguments.length; i++) newArgs.push(arguments[i]);

    newArgs[index] = argValue || someOtherFunction();

    return originalFunction.apply(this, newArgs);
  };
  return propDesc;
};


function someOtherFunction(): string {
  return "some value"
}

//---------------------
class MyClas {
 constructor() {} 
  
  @decoratorFunction(1) // the number is the required param index to be checked
  MyMethod(param1: string, param2?: string) {
    // if param2 is not provided the decorator will
    // "replace" it with the result of someOtherFunction()
  }
  
  @decoratorFunction(0) // the number is the required param index to be checked
  MyOtherMethod(param1?: string) {
    // if param1 is not provided the decorator will
    // "replace" it with the result of someOtherFunction()
  }  
}