Error with decorator in Typescript: Unable to resolve signature of property decorator when called as an expression.ts(1240)

Question

I have a problem when trying to create a simple decorator function on typescript. I don't know whats happening.

This is the decorator:

//Creating a decorator for loging the execution tyme of a method
export function logExecutionTime() {
  return function (
    target: any,
    propertyKey: string,
    descriptor: PropertyDescriptor
  ) {
    return <any>descriptor;
  };
}

and when I invoke it on another file like this

@logExecutionTime();

I get the following error:

Unable to resolve signature of property decorator when called as an expression.ts(1240)

If i don't force the type of the decorator's return to be any I get the previous error and also this one:

Decorator function return type is 'PropertyDescriptor' but is expected to be 'void' or 'any'.ts(1271)

Can someone please help? I'm just learning about decorators. Thanks!

I tried to create and invoke a simple decorator;

At the place where the decorator function is called gives this error: Unable to resolve signature of property decorator when called as an expression.

If i don't force the type of the decorator's return to be any I get the previous error and also this one:

Decorator function return type is 'PropertyDescriptor' but is expected to be 'void' or 'any'.ts(1271)

Answer 1

The inner function should not return a value. In a decorator, you change the decorated element by manipulating its description data. Have a look at this example:

export function logExecutionTime() {
  return function (
    target: any,
    propertyKey: string,
    descriptor: PropertyDescriptor  // this is the description we want to change
  ) {
    const originalFunction = descriptor.value // store the declared fuction 
    descriptor.value = function(...args){ // set a new function
        ... // set up your timer
        const result = originalFunction.apply(this, args) // run the declared function
        ... // process timer result
        return result
    }
  };
}