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Declaration Spaces

There are two declaration spaces in TypeScript: The variable declaration space and the type declaration space. These concepts are explored below.

Type Declaration Space

The type declaration space contains stuff that can be used as a type annotation. E.g the following are a few type declarations:

class Foo { }
interface Bar { }
type Bas = {}

This means that you can use Foo, Bar, Bas etc. as a type annotation. E.g.:

var foo: Foo;
var bar: Bar;
var bas: Bas;

Notice that even though you have interface Bar, you can't use it as a variable because it doesn't contribute to the variable declaration space. This is shown below:

interface Bar {};
var bar = Bar; // ERROR: "cannot find name 'Bar'"

The reason why it says cannot find name is because the name Bar is not defined in the variable declaration space. That brings us to the next topic "Variable Declaration Space".

Variable Declaration Space

The variable declaration space contains stuff that you can use as a variable. We saw that having class Foo contributes a type Foo to the type declaration space. Guess what?, it also contributes a variable Foo to the variable declaration space as shown below:

class Foo { }
var someVar = Foo;
var someOtherVar = 123;

This is great as sometimes you want to pass classes around as variables. Remember that

  • We couldn't use something like an interface that is only in the type declaration space as a variable.

Similarly something that you declare with var, is only in the variable declaration space and cannot be used as a type annotation:

var foo = 123;
var bar: foo; // ERROR: "cannot find name 'foo'"

The reason why it says cannot find name is because the name foo is not defined in the type declaration space.

TIPS

Copying Stuff around in the Type Declaration Space

If you want to move a class around you might be tempted to do the following:

class Foo { }
var Bar = Foo;
var bar: Bar; // ERROR: "cannot find name 'Bar'"

This is an error because var only copied the Foo into the variable declaration space and you therefore cannot use Bar as a type annotation. The proper way is to use the import keyword. Note that you can only use the import keyword in such a way if you are using namespaces or modules (more on these later):

namespace importing {
    export class Foo { }
}

import Bar = importing.Foo;
var bar: Bar; // Okay

Capturing the type of a variable

You can actually use a variable in a type annotation using the typeof operator. This allows you to tell the compiler that one variable is the same type as another. Here is an example to demonstrate this:

var foo = 123;
var bar: typeof foo; // `bar` has the same type as `foo` (here `number`)
bar = 456; // Okay
bar = '789'; // ERROR: Type `string` is not `assignable` to type `number`

Capturing the type of a class member

Similar to capturing the type of a variable, you just declare a variable purely for type capturing purposes:

class Foo {
  foo: number; // some member whose type we want to capture
}

// Purely to capture type
declare let _foo: Foo;

// Same as before
let bar: typeof _foo.foo;