TypeScript parameter and metadata decorators example

parameter_decorators_part_1.ts

"use strict";

function logParameter(target:Object, key:string, index:number) {
	var metadataKey = `log_${key}_parameters`;
	if (Array.isArray(target[metadataKey])) {
		target[metadataKey].push(index);
	}
	else {
		target[metadataKey] = [index];
	}
}

function logMethod(target:Object, key:string, descriptor:any) {

	var originalMethod = descriptor.value;

	descriptor.value = function (...args:any[]) {

		var indices = target[`__log_${key}_parameters`];

		if (Array.isArray(indices)) {
			for (let i = 0; i < args.length; i++) {
				if (indices.indexOf(i) !== -1) {
					let arg = args[i];
					let argStr = JSON.stringify(arg) || arg.toString();
					console.log(`${key} arg[${i}]: ${argStr}`);
				}
			}

			return originalMethod.apply(this, args);
		}


		let a = args.map(a => (JSON.stringify(a) || a.toString())).join();
		let result = originalMethod.apply(this, args);
		let cln = target.constructor.name;

		console.log(`Call: ${cln}.${key}(${a}) => ${JSON.stringify(result)}`);

		return result;
	};

	return descriptor;
}


class Person {

	constructor(
		public name:string,
		public surname:string
	){}

	@logMethod
	public say(@logParameter message:string):string {
		return `${this.name} ${this.surname} says: ${message}`;
	}
}

let me = new Person("Alex", "Mayorov");
let you = new Person("Kira", "Hagakyre");
me.say('Hi, Kira!');
you.say('Hi, Alex!');

/* Output:

Call: Person.say("Hi, Kira!") => "Alex Mayorov says: Hi, Kira!"
Call: Person.say("Hi, Alex!") => "Kira Hagakyre says: Hi, Alex!"

*/