Typescript initialize static variable of a class type

Question

I have two classes Foo and Bar. In class Bar I have a static variable called myFoo and I want it to be automatically initialized:

class Foo {
}

class Bar {
    static myFoo: Foo = new Foo();
}

However, I'm getting this error:

Uncaught ReferenceError: Foo is not defined

If I initialize that static variable in Bar's constructor then it works fine:

class Bar {
    static myFoo: Foo;

    constructor() {
         Bar.myFoo = new Foo();
    }
}

Why is that? What did I do wrong when I tried to initialize the static variable myFoo directly?

The code above works fine, and static myFoo: Foo = new Foo(); is indeed the correct way to initialize it. As explained in the accepted answer, the problem seems to be related to import order when Foo and Bar are in different files. (Just commenting in case anybody else is confused by the question.) — Jo Liss
– Jo Liss, Commented Mar 23, 2024 at 16:07

Jeffery Grajkowski · Accepted Answer · 2014-06-20 16:28:56Z

9

You definitely don't want to do that second thing because that's going to overwrite myFoo every time you construct a new Bar and you certainly don't want that.

What you have here is a run time problem, not a compile time problem. The Foo class has to be loaded before the Bar class is loaded otherwise the static initializer will fail. If both classes are in a single file in the above order it works. If the classes are in separate files and you tell TypeScript to compile to a single file it should figure out the correct order for you (though there are bugs in that area). If you're compiling to separate files you'll need to include the scripts on the page in the correct order to satisfy the dependency.

answered Jun 20, 2014 at 16:28

Jeffery Grajkowski

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1 Comment

Chin Over a year ago

indeed, it was because I imported the script files in the wrong order. Thanks!

Oded Breiner · Accepted Answer · 2017-07-11 06:54:35Z

7

You can just have the call to initialize immediately follow the class declaration:

class MyClass {
    static initialize() {
        // Initialization
    }
}
MyClass.initialize();

answered Jul 11, 2017 at 6:54

Oded Breiner

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Comments

Karthik Sankar · Accepted Answer · 2021-02-17 20:30:05Z

0

As Jeffery Grajkowski said, your second approach is incorrect. You could just initialize in definition.

class Bar {
  static myFoo: Foo = new Foo();
}

edited Feb 17, 2021 at 20:30

answered Feb 17, 2021 at 20:25

Karthik Sankar

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Comments

Dave Cousineau · Accepted Answer · 2017-07-22 21:59:19Z

-1

This requires JQuery, but is what I do to have the equivalent of a 'static constructor'.

namespace SomeNamespace {
   $(() => SomeClass.StaticConstructor());

   export class SomeClass {
      public static StaticConstructor() {
      }
   }
}

This is also useful as the "entry point" of your application, for example.

namespace SomeNamespace {
   $(() => SomeClass.Start());

   export class SomeClass {
      private static sInstance: SomeClass;

      public static Start() {
         SomeClass.sInstance = new SomeClass();
      }
   }
}

answered Jul 22, 2017 at 21:59

Dave Cousineau

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