function_decorators.ts

// Debug/Checked compilation 
const DEBUG = true;

function conditional(condition) {
    return target => condition ? target : Function.prototype;
} 

@conditional(DEBUG)
function assert(condition, message = "assertion failed.") {
    if (!condition) throw new Error(message);
}

// Conditional decorators

@conditional(TRACE)
function log(message) {
    return target => function () {
       console.log(message);
       return target.apply(this, arguments);
    };
}

@log("entering main()") // only when TRACE is true
function main() { }

// Design-time metadata
@metadata("design:paramtypes", () => [Number, Number])
@metadata("design:returntype", () => Number)
function add(a, b) { 
    return a + b;
}

// Testing
@test function addIsCorrect() {
    const a = 1, b = 1;
    const expected = 2;
    let result = add(a, b);
    assert(result === expected);
}

// API deprecation
function obsolete(message, isError) {
    return target => function () {
        if (isError) throw new Error(message);
        console.warn(message);
        return target.apply(this, arguments);
    };
}

@obsolete("This API is obsolete and will be removed in v2")
export function oldAPIFunction() {
}

// Dependency Injection/IoC
import { WidgetFactory, PartBuilder, Fabricator } from "service-symbols";

@parameter(0, inject(PartBuilder))
@parameter(1, inject(Fabricator))
function widgetFactory(partBuilder, fabricator) {
    return fabricator.fabricate(partBuilder.build());
}

const container = new DIContainer();
container
    .for(PartBuilder).use(concretePartBuilder)
    .for(Fabricator).use(concreteFabricator)
    .for(WidgetFactory).use(widgetFactory)
    
let factory = container.get(WidgetFactory);
let widget = factory();

// Diagnostics/Performance
@measure("parse")
@log({ category: "parse" })
function parse(tokens) {
    // ...
}

@measure("emit")
@log({ category: "emit" })
function emit(ast, out) {
    // ...
}

// ...

console.log("parse time: " + measure.for("parse"));
console.log("emit time: " + measure.for("emit"));

// Others/Future extensibility...
@futurePrototocolOnTopOfGenerators function* fancy() {}

@curryify function add(a, b) { }

@pure function f() { }

@permission("unrestricted") function accessLocker() { }

@operator(Point, "+", Point) function addPointAndPoint(p1, p2) { }