Call typescript decorator method when the underlying function is executed

Question

As per how typescript design works ,if i write a decorator like,

class foo{

   @fooDecorator
   public fooMethod(){

    }
}

it gives the transpiled javascript as

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
return c > 3 && r && Object.defineProperty(target, key, r), r;
 };
var foo = (function () {
function foo() {
}
foo.prototype.fooMethod = function () {
};
__decorate([
    fooDecorator
], foo.prototype, "fooMethod", null);
return foo;
}());

as per there transpiled code whenever my app gets bootstrapped the decorator function get executed, but how i want is whenever the underlying function "fooMethod" is executed the decorator method "fooDecorator" should execute, how can i attain the same, please help me with the issue

Answer 1

You can do it like this. In the decorator replace your original method with 'wrapper' that will first call the supplied method that should be called each time and then call the original one.

The code below illustrate this approach:

Your decorator:

export function Foo(funcToCallEveryTime: (...args: any[]) => void)
{
    return (target: any, key: string, descriptor: any) => 
    {
        var originalMethod = descriptor.value; 

        descriptor.value =  function (...args: any[]) {
            funcToCallEveryTime(...args);
            return originalMethod.apply(target, args);
        }

        return descriptor;
    }
}

And method you decorate with @Foo decorator like this:

@Foo((...args: any[]) => { console.log("Before method call:", args[0]); })
private TestMethod(param: string): void
{
    console.log("Method call");
    //...
}

Hope this helps.