get and set in TypeScript

Typescript uses getter/setter syntax that is like ActionScript3.

class foo {
    private _bar: boolean = false;
    get bar(): boolean {
        return this._bar;
    }
    set bar(value: boolean) {
        this._bar = value;
    }
}

That will produce this Javascript, using the Ecmascript 5 Object.defineProperty() feature.

var foo = (function () {
    function foo() {
        this._bar = false;
    }
    Object.defineProperty(foo.prototype, "bar", {
        get: function () {
            return this._bar;
        },
        set: function (value) {
            this._bar = value;
        },
        enumerable: true,
        configurable: true
    });
    return foo;
})();

So to use it,

var myFoo = new foo();
if(myFoo.bar) {         // calls the getter
    myFoo.bar = false;  // calls the setter and passes false
}

However, in order to use it at all, you must make sure the TypeScript compiler targets ECMAScript5. If you are running the command line compiler, use --target flag like this;

tsc --target ES5

If you are using Visual Studio, you must edit your project file to add the flag to the configuration for the TypeScriptCompile build tool. You can see that here:

As @DanFromGermany suggests below, if your are simply reading and writing a local property like foo.bar = true, then having a setter and getter pair is overkill. You can always add them later if you need to do something, like logging, whenever the property is read or written.

Ezward has already provided a good answer, but I noticed that one of the comments asks how it is used. For people like me who stumble across this question, I thought it would be useful to have a link to the official documentation on getters and setters on the Typescript website as that explains it well, will hopefully always stay up-to-date as changes are made, and shows example usage:

http://www.typescriptlang.org/docs/handbook/classes.html

In particular, for those not familiar with it, note that you don't incorporate the word 'get' into a call to a getter (and similarly for setters):

var myBar = myFoo.getBar(); // wrong    
var myBar = myFoo.get('bar');  // wrong

You should simply do this:

var myBar = myFoo.bar;  // correct (get)
myFoo.bar = true;  // correct (set) (false is correct too obviously!)

given a class like:

class foo {
  private _bar:boolean = false;

  get bar():boolean {
    return this._bar;
  }
  set bar(theBar:boolean) {
    this._bar = theBar;
  }
}

then the 'bar' getter for the private '_bar' property will be called.

Here's a working example that should point you in the right direction:

class Foo {
    _name;

    get Name() {
        return this._name;
    }

    set Name(val) {
        this._name = val;
    }
}

Getters and setters in JavaScript are just normal functions. The setter is a function that takes a parameter whose value is the value being set.

You can write this

class Human {
    private firstName : string;
    private lastName : string;

    constructor (
        public FirstName?:string, 
        public LastName?:string) {

    }

    get FirstName() : string {
        console.log("Get FirstName : ", this.firstName);
        return this.firstName;
    }
    set FirstName(value : string) {
        console.log("Set FirstName : ", value);
        this.firstName = value;
    } 

    get LastName() : string {
        console.log("Get LastName : ", this.lastName);
        return this.lastName;
    }
    set LastName(value : string) {
        console.log("Set LastName : ", value);
        this.lastName = value;
    } 

}

It is very similar to creating common methods, simply put the keyword reserved get or set at the beginning.

class Name{
    private _name: string;

    getMethod(): string{
        return this._name;
    }

    setMethod(value: string){
        this._name = value
    }

    get getMethod1(): string{
        return this._name;
    }

    set setMethod1(value: string){
        this._name = value
    }
}

class HelloWorld {

    public static main(){

        let test = new Name();

        test.setMethod('test.getMethod() --- need ()');
            console.log(test.getMethod());

        test.setMethod1 = 'test.getMethod1 --- no need (), and used = for set ';
            console.log(test.getMethod1);
    }
}
HelloWorld.main();

In this case you can skip return type in get getMethod1() {

    get getMethod1() {
        return this._name;
    }

I think I probably get why is it so confusing. In your example, we wanted getters and setters for _name. But we achieve that by creating getters and setters for an unrelated class variable Name.

Consider this:

class Car{
    private tiresCount = 4;
    get yourCarTiresCount(){
        return this.tiresCount ;
    }
    set yourCarTiresCount(count) {
        alert('You shouldn't change car tire count')
    }
}

Above code does following:

1. get and set create getter and setter for yourCarTiresCount (not for tiresCount).

The getter is :

function() {
    return this.tiresCount ;
}

and the setter is :

function(count) {
    alert('You shouldn't change car tire count');
}

Meaning, every time we do new Car().yourCarTiresCount, getter runs. And for every new Car().yourCarTiresCount('7') setter runs.

2. Indirectly create getter, but not the setter, for private tireCount.

TS offers getters and setters which allow object properties to have more control of how they are accessed (getter) or updated (setter) outside of the object. Instead of directly accessing or updating the property a proxy function is called.

Example:

class Person {
    constructor(name: string) {
        this._name = name;
    }

    private _name: string;

    get name() {
        return this._name;
    }

    // first checks the length of the name and then updates the name.
    set name(name: string) {
        if (name.length > 10) {
            throw new Error("Name has a max length of 10");
        }

        this._name = name;  
    }

    doStuff () {
        this._name = 'foofooooooofoooo';
    }


}

const person = new Person('Willem');

// doesn't throw error, setter function not called within the object method when this._name is changed
person.doStuff();  

// throws error because setter is called and name is longer than 10 characters
person.name = 'barbarbarbarbarbar';  

If you are working with TypeScript modules and are trying to add a getter that is exported, you can do something like this:

// dataStore.ts
export const myData: string = undefined;  // just for typing support
let _myData: string;  // for memoizing the getter results

Object.defineProperty(this, "myData", {
    get: (): string => {
        if (_myData === undefined) {
            _myData = "my data";  // pretend this took a long time
        }

        return _myData;
    },
});

Then, in another file you have:

import * as dataStore from "./dataStore"
console.log(dataStore.myData); // "my data"