when are typescript/js decorators executed

Question

When do decorators get executed?

class Person {
    @SomeDecorator
    age
}

1. When I create an instance of Person
2. When the Person class is parsed

What about static properties?

Answer 1

A property decorator executes early - when the class is defined. You don't need to construct an instance, or access the property.

Example: this logs age without the Person class even being constructed. The same applies if the property is static.

function SomeDecorator(a, b) {
    console.log(b);
}

class Person {
    @SomeDecorator
    public age: number;
}

If you are after hooking into the get and set actions on the property - that is possible too. Here is an example from a listing in Pro TypeScript (Second Edition). It works by wrapping the getter and setter.

function log(target: any, key: string) {
    let value = target[key];

    // Replacement getter
    const getter = function () {
        console.log(`Getter for ${key} returned ${value}`);
        return value;
    };

    // Replacement setter
    const setter = function (newVal) {
        console.log(`Set ${key} to ${newVal}`);
        value = newVal;
    };

    // Replace the property
    if (delete this[key]) {
        Object.defineProperty(target, key, {
            get: getter,
            set: setter,
            enumerable: true,
            configurable: true
        });
    }
}

class Calculator {
    @log
    public num: number;

    square() {
        return this.num * this.num;
    }
}

console.log('Construct');
const calc = new Calculator();

console.log('Set');
// Set num to 4
calc.num = 4;

console.log('Get');
// Getter for num returned 4
// Getter for num returned 4
calc.square();

The output of this listing is:

Construct (manual log)

Set (manual log)

-> Set num to 4

Get (manual log)

-> Getter for num returned 4

-> Getter for num returned 4