[TypeScript] Understanding Decorators

Decorators are a feature of TypeScript that are becoming more and more common in many major libraries. This lesson walks you through what decorators are and how to create your own.

Nomarl way to decorate a object :

const Person = {name: ‘John‘};

function addAge(age){
    return function(person){
        return {
            age,
            name: person.name
        }
    }
}

const john = addAge(30)(Person);
console.log(john); // {name: "John", age: 30}

In Typescript, we can enable "experimentaDecorators", which is ES7 feature:

{
    "compilerOptions": {
        "rootDir": "src",
        "module": "commonjs",
        "target": "es5",
        "noImplicitAny": false,
        "sourceMap": false,
        "outDir": "./dist",
        "noEmitOnError": true,
        "experimentalDecorators": true
    },
    "exclude": [
        "node_modules",
        "typings/main",
        "typings/main.d.ts"
    ]
}

function addAge(age){
    return function(targetClass){
        return class{
            age = age;
            name = new targetClass().name;
        }
    }
}

@addAge(30)
class Person{
    name = "Johnn";
}

const john = new Person();
console.log(john); // {name: "Johnn", age: 30}

As before we create addAge function as decorator, different from before, it return class, because we want to decorate Person class.

After the decorator, we will have age prop on the person class.