Typescript Private Method

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JavaScript has drastically improved as a language over the past few years. TypeScript provides some nice features on top of the JavaScript such as static typing. In this post, we are going to cover one of these TypeScript features, the private keyword.
Before we dive into TypeScript's private feature let's do a quick recap of JavaScript classes. In the new ES2015 standard of JavaScript we get a Object Oriented Class style syntax that looks like the following,
In this example we are using pure JavaScript no TypeScript syntax or features are being used. JavaScript classes can be exported and used in other JavaScript modules. JavaScript classes also have constructors, properties, and methods similar to most Class-based languages we see today. Unfortunately, in the current version of JavaScript, there is no support for private properties or private methods yet. In JavaScript all class instance properties and methods are public.
Using TypeScript, we can add private functionality into our classes. What are private properties or methods? A private property of method can only be accessed or called from the class instance itself. Let's take a look at an example private property.
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We can see the private hides the property from the user/consumer of the class. If I try to set cory.age we also get an error because I defined only a get for the age property with no set so this property can only be read-only. We can go one step further and refactor our constructor a little bit more.
In this example, we can simplify our constructor parameters by combining the declaration and assignment into a single statement.
We can prefix the constructor parameters with the public or private keyword to automatically have TypeScript assign the parameter as a property of the class. In this example, this removes the unnecessary declaration and assignment of both firstName and lastName.
Methods can also be private which is useful for hiding implementation detail of how a Class works to the user of the Class. Let's take a look at a minimal example.
In this example, we created a private method log(). Log can only be called by other methods in our class. You can see above if I try to call log directly, we get a TypeScript error. Private properties and methods can help enforce logic for when data is updated and also enforce encapsulation of our classes. Check out the full working demo in the link below!
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Is it possible to create a private "function" (method) within a TypeScript class? Let's assume we have the following Person.ts TypeScript file:
Which when compiled is being transformed to the following:
I was expecting the whisper function to be declared within the closure, but not on the prototype? Essentially this makes the whisper function public when compiled?
Richard
Richard 7,762●33 gold badges●3232 silver badges●5757 bronze badges
TypeScript public/private keywords only apply to the way TypeScript checks your code - they don't have any effect on the JavaScript output.
Private visibility is a design-time construct; it is enforced during static type checking but does not imply any runtime enforcement. ... TypeScript enforces encapsulation of implementation in classes at design time (by restricting use of private members), but cannot enforce encapsulation at runtime because all object properties are accessible at runtime. Future versions of JavaScript may provide private names which would enable runtime enforcement of private members
This has already been asked and answered here: https://stackoverflow.com/a/12713869/1014822
Update: This old answer still gets an amount of traffic, so worth noting that, in addition to the language spec link above, public, private, and (now) protected members are covered in detail in the TypeScript handbook chapter on classes.
2018 Update Implementation of ES Private Fields is now a Future item on the TypeScript RoadMap although discussion suggests this will be a parallel hard private option, not a replacement for the current soft private implementation.
2020 Update Runtime private fields using the # operator have been implemented as of TS 3.8. There is a good discussion of how they work and how they differ from compile-time fields with the private keyword on StackOverflow here.
Private methods have reached stage 3 of the TC39 working group. The feature is currently in active discussion for TypeScript, for example here.
Jude Fisher
Jude Fisher 10.8k●66 gold badges●4545 silver badges●8686 bronze badges
Odd that the chapter on functions is silent on this. – StingyJack Mar 29 '17 at 22:33
In Javascript (as opposed to TypeScript), you can't have a private "member" function.
If you define a private function in the closure, you won't be able to call it as an instance method on an instance of your class.
If that's what you want, just move the TypeScript function definition outside the class body.
SLaks
SLaks 811k●167167 gold badges●18211821 silver badges●19041904 bronze badges
Hmm, I guess there where the var self = this; applies? Is it possible to declare a function within a closure that doesn't explicitly access any instance information? – Richard Jun 4 '13 at 13:49
@Richard: No; var self = this has nothing to do with this. The point is that there is no way to make instance.method() only work in your closure. – SLaks Jun 4 '13 at 13:51
Sure; but it is possible to simply call method() from within an instance (public) method, is it not? – Richard Jun 4 '13 at 13:53
Yes, but it will have nothing to do with the class. (although you can accept this as a parameter) – SLaks Jun 4 '13 at 13:54
It can be done, but every call to a private method would have to be done using apply() or call() with the instance "this". – Nomaed Jan 19 '16 at 7:52
You can use private class variables and functions in TypeScript/Webpack and even in the latest Google Chrome versions natively. Just add a # before the name. See here.
air5
air5 71●22 silver badges●77 bronze badges
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