Decorators
Stage 3 (Finally!)
What is a decorator?
function add(a, b) {
console.log(`calling add, args:`, ...args);
const result = a + b;
console.log(`finished add, result:`, result);
return result;
}
function sub(a, b) {
console.log(`calling sub, args:`, ...args);
const result = a - b;
console.log(`finished sub, result:`, result);
return result;
}function debugLog(fn) {
return (...args) => {
console.log(`calling ${fn.name}, args:`, ...args);
const result = fn(...args);
console.log(`finished ${fn.name}, result:`, result);
return result;
};
}
const add = debugLog((a, b) => a + b);
const sub = debugLog((a, b) => a - b);const C = decorate1(
decorate2(
class {
// ...
}
)
);class C {
// ???
method() {}
}Decorators as of March 2022
Decorators as of March 2022
function dec(value, context) {
// apply decoration
}Decorators as of March 2022
function dec(value, context) {
// apply decoration
}
@dec
class C {
@dec m() {}
@dec get x() {}
@dec set x(v) {}
@dec y = 123;
@dec accessor z = 456;
}Decorators as of March 2022
function dec(value, context) {
// apply decoration
}
@dec
class C {
@dec m() {}
@dec get x() {}
@dec set x(v) {}
@dec y = 123;
@dec accessor z = 456;
}
@second
@first
class C {
@second @first x = 123;
}function dec(Class) {
return class Decorated extends Class {
// ...
}
}
@dec class C {}function logResult(method) {
return function (...args) {
const result = method.apply(this, args);
console.log(result);
return result;
}
}
class C {
@logResult m() {
return 123;
}
}
new C().m(); // logs "123"function logResult(method) {
return function (...args) {
const result = method.apply(this, args);
console.log(result);
return result;
}
}
class C {
@logResult get x() {
return 123;
}
}
new C().x; // logs "123"function add1() {
return function (value) {
return value + 1;
}
}
class C {
@add1 x = 1;
}
console.log(new C().x); // 2class C {
accessor x = 123;
}
class C {
accessor x = 123;
}
// is approximately the same as
class C {
#x = 123;
get x() {
return this.#x;
}
set x(value) {
this.#x = value;
}
}function addOneAndLog({ get, set }) {
return {
get() {
const value = get.call(this);
console.log('getting value:', value);
return value;
},
set(value) {
console.log('setting value:', value + 1)
set.call(this, value + 1);
},
init(value) {
return value + 1;
}
}
}
class C {
@addOneAndLog accessor x = 1;
}
const c = new C();
c.x; // getting value: 2;
c.x = 2; // setting value: 3;interface DecoratorContext {
kind: 'class' | 'method' | 'getter' | 'setter' | 'field' | 'accessor';
name: string | symbol;
private?: boolean;
static?: boolean;
access: {
get?(): unknown;
set?(value: unknown): void;
};
addInitializer?(initializer: () => void): void;
}function bound(_, { access, addInitializer }) {
addInitializer(function () {
// get the final method
const method = access.get.call(this);
// set it to a bound version
access.set.call(this, method.bind(this));
});
}
class C {
@bound m() {}
}class C {
@inject('foo') bar;
}function inject(name) {
return function injectionDecorator() {
// ...injection logic using name
}
}
class C {
@inject('foo') bar;
}class C {
// valid
@foo.bar x;
@foo.bar() x;
@(foo['bar']().baz) x;
// invalid
@foo['bar'] x;
@foo().bar() x;
}Major Differences
Between March 2022 and Babel Legacy/Typescript
1. Decorators cannot change the type of a decorated value.
2. Decorators cannot change the property descriptor for class elements.
3. Class element decorators do not have access to the class itself.
4. Class decorator come after the export keyword
4. Class decorator come after the export keyword
// before
@dec export class Foo {
}
// after
export @dec class Foo {
}
function makeAClass() {
return @dec class Bar {
// ...
}
}5. Decorator expressions are more restricted.
5. Decorator expressions are more restricted.
// before
export class Foo {
@foo().bar() x;
}
// after
export @dec class Foo {
@(foo().bar()) x
}
5. Decorator expressions are more restricted.
4. Class decorator come after the export keyword
3. Class element decorators do not have access to the class itself.
2. Decorators cannot change the property descriptor for class elements.
1. Decorators cannot change the type of a decorated value.
Major Differences
Looking Forward
Thank you!
pzuraq.com
Decorators Stage 3 (Finally!)
By pzuraq
