How do I write an extension method in JavaScript?

JavaScript doesn't have an exact analogue for C#'s extension methods. JavaScript and C# are quite different languages.

The nearest similar thing is to modify the prototype object of all string objects: String.prototype. In general, best practice is not to modify the prototypes of built-in objects in library code meant to be combined with other code you don't control. (Doing it in an application where you control what other code is included in the application is okay.)

If you do modify the prototype of a built-in, it's best (by far) to make that a non-enumerable property by using Object.defineProperty (ES5+, so basically any modern JavaScript environment, and not IE8¹ or earlier). To match the enumerability, writability, and configurability of other string methods, it would look like this:

Object.defineProperty(String.prototype, "SayHi", {
    value: function SayHi() {
        return "Hi " + this + "!";
    },
    writable: true,
    configurable: true,
});

(The default for enumerable is false.)

If you needed to support obsolete environments, then for String.prototype, specifically, you could probably get away with creating an enumerable property:

// Don't do this if you can use `Object.defineProperty`
String.prototype.SayHi = function SayHi() {
    return "Hi " + this + "!";
};

That's not a good idea, but you might get away with it. Never do that with Array.prototype or Object.prototype; creating enumerable properties on those is a Bad Thing™.

Details:

JavaScript is a prototypical language. That means that every object is backed by a prototype object. In JavaScript, that prototype is assigned in one of four ways:

• By the constructor function for the object (e.g., new Foo creates an object with Foo.prototype as its prototype)
• By the Object.create function added in ES5 (2009)
• By the Object.setPrototypeOf function (ES2015+) [or the deprecated __proto__ setter (ES2015+, optional, and only exists on objects that inherit [directly or indirectly] from Object.prototype), or
• By the JavaScript engine when creating an object for a primitive because you're calling a method on it (this is sometimes called "promotion")

So in your example, since firstName is a string primitive, it gets promoted to a String instance whenever you call a method on it, and that String instance's prototype is String.prototype. So adding a property to String.prototype that references your SayHi function makes that function available on all String instances (and effectively on string primitives, because they get promoted).

Example:

Object.defineProperty(String.prototype, "SayHi", {
    value: function SayHi() {
        return "Hi " + this + "!";
    },
    writable: true,
    configurable: true
});

console.log("Charlie".SayHi());

There are some key differences between this and C# extension methods:

• (As DougR pointed out in a comment) C#'s extension methods can be called on null references. If you have a string extension method, this code:

    string s = null;
    s.YourExtensionMethod();
  

  works (unless YourExtensionMethod throws when it receives null as the instance parameter). That isn't true with JavaScript; null is its own type, and any property access on null throws an error. (And even if it didn't, there's no prototype to extend for the Null type.)

• (As ChrisW pointed out in a comment) C#'s extension methods aren't global. They're only accessible if the namespace they're defined in is used by the code using the extension method. (They're really syntactic sugar for static calls, which is why they work on null.) That isn't true in JavaScript: If you change the prototype of a built-in, that change is seen by all code in the entire realm you do that in (a realm is the global environment and its associated intrinsic objects, etc.). So if you do this in a web page, all code you load on that page sees the change. If you do this in a Node.js module, all code loaded in the same realm as that module will see the change. In both cases, that's why you don't do this in library code. (Web workers and Node.js worker threads are loaded in their own realm, so they have a different global environment and different intrinsics than the main thread. But that realm is still shared with any modules they load.)

¹ IE8 does have Object.defineProperty, but it only works on DOM objects, not JavaScript objects. String.prototype is a JavaScript object.

Every object has a parent (prototype), you can prove this by logging any object to the console and you'll see a prototype object, you can expand the prototype object to see all the methods and properties (use dev tools in your browser). The example below will add a new method to the Array prototype that will get inherited.

Array.prototype.hello = function() {
        console.log('hello')
}