How to implement class constants?

Question

In TypeScript, the const keyword cannot be used to declare class properties. Doing so causes the compiler to an error with "A class member cannot have the 'const' keyword."

I find myself in need to clearly indicate in code that a property should not be changed. I want the IDE or compiler to error if I attempt to assign a new value to the property once it has been declared. How do you guys achieve this?

I'm currently using a read-only property, but I'm new to Typescript (and JavaScript) and wonder whether there is a better way:

get MY_CONSTANT():number {return 10}; 

I'm using typescript 1.8. Suggestions?

PS: I'm now using typescript 2.0.3, so I've accepted David's answer

Answer 1

TypeScript 2.0 has the readonly modifier:

class MyClass {     readonly myReadOnlyProperty = 1;      myMethod() {         console.log(this.myReadOnlyProperty);         this.myReadOnlyProperty = 5; // error, readonly     } }  new MyClass().myReadOnlyProperty = 5; // error, readonly 

It's not exactly a constant because it allows assignment in the constructor, but that's most likely not a big deal.

Alternative Solution

An alternative is to use the static keyword with readonly:

class MyClass {     static readonly myReadOnlyProperty = 1;      constructor() {         MyClass.myReadOnlyProperty = 5; // error, readonly     }      myMethod() {         console.log(MyClass.myReadOnlyProperty);         MyClass.myReadOnlyProperty = 5; // error, readonly     } }  MyClass.myReadOnlyProperty = 5; // error, readonly 

This has the benefit of not being assignable in the constructor and only existing in one place.

Answer 2

Constants can be declare outside of classes and use within your class. Otherwise the get property is a nice workaround

const MY_CONSTANT: string = "wazzup";  export class MyClass {      public myFunction() {          alert(MY_CONSTANT);     } }