Consider a scenario where you need to dynamically modify method arguments using a decorator at runtime. To illustrate this concept, let's simplify it with an example: setting all arguments to "Hello World":
export const SillyArguments = (): MethodDecorator => {
return (
target: Object,
propertyKey: string | symbol,
descriptor: PropertyDescriptor
) => {
const originalMethod = descriptor.value;
descriptor.value = (...args: any[]) => {
Object.keys(args).forEach(i => {
args[i] = 'Hello World';
});
return originalMethod.apply(null, args);
};
return descriptor;
}
};
Here is how you can use this decorator:
class TestClass {
private qux = 'qux';
@SillyArguments()
foo(val: any) {
console.log(val);
console.log(this.qux);
this.bar();
}
bar() {
console.log('bar');
}
}
const test = new TestClass();
test.foo('Ciao mondo'); // prints "Hello World"
TypeError: Cannot read property 'qux' of null
The issue arises from apply(null, args), which changes the context of this. Thus, accessing the instance variable qux within foo() becomes impossible.
One option is to modify the call to
originalMethod.apply(target, args)qux is undefined when invoking foo().
Is there a way to execute originalMethod with the correct context of this set to the instance?