Typescript Private Method
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My name is Cory Rylan .
Google Developer Expert , speaker, and
Software Developer at VMware Clarity Design System .
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.
In this example, we are using a typical pattern of a private property to control how a property is updated. In our use case, it is valid to increase the age of a person, but you cannot set it to a random value or a younger age. To enforce this, we create a private property _age . The _age property is a property that will be only available internally to the class. For example, if I try to set _age I get an error because it is private to the class. If I try to read _age I also get the same error
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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Last updated: March 3, 2020 by Valentino Gagliardi - 4 minutes read
class Person {
#age ;
#name ;
#surname ;
constructor ( name , surname , age ) {
this . #name = name ;
this . #surname = surname ;
this . #age = age ;
}
getFullName ( ) {
return ` ${ this . #name } + ${ this . #surname } ` ;
}
}
class Person {
#age ;
#name ;
#surname ;
constructor ( name , surname , age ) {
this . #name = name ;
this . #surname = surname ;
this . #age = age ;
}
getFullName ( ) {
return ` ${ this . #name } + ${ this . #surname } ` ;
}
}
const marta = new Person ( "Marta" , "Cantrell" , 33 ) ;
console . log ( marta . #age ) ; // SyntaxError
class Person {
private age : number ;
private name : string ;
private surname : string ;
constructor ( name : string , surname : string , age : number ) {
this . name = name ;
this . surname = surname ;
this . age = age ;
}
getFullName ( ) {
return ` ${ this . name } + ${ this . surname } ` ;
}
}
const liz = new Person ( "Liz" , "Cantrill" , 31 ) ;
// @ts-ignore
console . log ( liz . age ) ;
"use strict" ;
var Person = /** @class */ ( function ( ) {
function Person ( name , surname , age ) {
this . name = name ;
this . surname = surname ;
this . age = age ;
}
Person . prototype . getFullName = function ( ) {
return this . name + " + " + this . surname ;
} ;
return Person ;
} ( ) ) ;
var liz = new Person ( "Liz" , "Cantrill" , 31 ) ;
console . log ( liz . age ) ; // 31
class Person {
#age : number ;
#name : string ;
#surname : string ;
constructor ( name : string , surname : string , age : number ) {
this . #name = name ;
this . #surname = surname ;
this . #age = age ;
}
getFullName ( ) {
return ` ${ this . #name } + ${ this . #surname } ` ;
}
}
{
"compilerOptions" : {
"target" : "es2015" ,
"strict" : true ,
"lib" : [ "dom" , "es2015" ]
}
}
In this post we'll shed light on ECMAScript private fields and see how they compare to the TypeScript private modifier.
A closure is the only JavaScript native mechanism for protecting variables from access .
Closures are the foundation for a lot of private-like patterns, like the popular module pattern . But after ECMAScript 2015 classes took over in recent years, developers felt the need for more control over classes member privacy .
The class field proposal (at the time of writing in stage 3) tries to solve the problem with the introduction of private class fields .
Here's a JavaScript class with private fields, note that unlike "public" members every private field must be declared before access :
Private class fields are not accessible from outside the class :
This is true "privacy". At this point if you now a bit of TypeScript you might ask what "native" private fields have in common with the private modifier in TypeScript .
Well, the answer is: nothing . But why?
The private modifier in TypeScript should be familiar to developers coming from more traditional backgrounds. In brief, the keyword is meant to deny class members access from outside the class.
But let's not forget, TypeScript is a layer on top of JavaScript and the TypeScript compiler is supposed to strip away any fancy TypeScript annotation, including private .
That means the following class doesn't do what you think it does:
Without // @ts-ignore , accessing liz.age throws an error only in TypeScript , yet after the compilation you end up with the following JavaScript code:
As expected we're free to print Liz's age. The main take here is that private in TypeScript is not so private , and it feels convenient only at the TypeScript level, not for "real privacy".
And now let's get to the point: "native" private class fields in TypeScript .
TypeScript 3.8 added support for ECMAScript private fields , not to be confused with the TypeScript private modifier .
Here's a class with private class fields in TypeScript:
Not so different from vanilla JavaScript, besides type annotations. Members cannot be accessed from the outside. But the real problem with private fields in TypeScript is that they use WeakMap under the hood.
To compile this code we need to adjust the target compilation version in tsconfig.json, which must be at least ECMAScript 2015 :
This could be a problem depending on the target browser, unless you intend to ship a polyfill for WeakMap, which at that point becomes too much work if it's just for the sake of writing fancy new syntax .
There is always this tension in JavaScript, where you really want to use the new syntax, but on the other hand you don't want to let the UX down with a gazillion polyfills.
On the flip side I don't think you should worry too much about private class fields, even if you want to ship to newer browsers. At least for now. The support for private fields is almost non-existent . Not even Firefox has implemented the proposal.
Still a proposal at the time of writing, JavaScript class fields are interesting, but the support among browser vendors is poor. What's your take on this feature ?
Hi! I’m Valentino! Educator and consultant, I help people learning to code with on-site and remote workshops. Looking for JavaScript and Python training? Let’s get in touch!
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https://coryrylan.com/blog/private-methods-and-properties-in-typescript-classes
https://www.valentinog.com/blog/private/
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