TypeScript 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.

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;
}
}

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