Property does not exist on globalThis immediately after declaring it

[weswigham]

Code

declare module "buf" {
    global {
        class Buffer {}
    }

    import Buf = globalThis.Buffer;
    export { Buf as Buffer };
}

Expected behavior:
No errors.

Actual behavior:
Property 'Buffer' does not exist on type 'typeof globalThis'.

Playground Link

[ExE-Boss]

That’s because class, let and const don’t create own properties on the global object, instead they create variable bindings on a Declarative Environment Record, so this is the expected behaviour.

This is also why microsoft/TypeScript-DOM-lib-generator#858 doesn’t quite work the way I intended.

[weswigham]

😢 Hmm, in this case we actually want a varlike global class, though, hrm

[ExE-Boss]

How about supporting var class <name> in ambient contexts?

declare module "buf" {
	global {
		var class Buffer {}
	}

	import Buf = globalThis.Buffer;
	export { Buf as Buffer };
}

[trusktr]

To explain this differently for other people, this behavior is because it works like the following in actual browser script tags (assuming that's what the TypeScript global declarations are emulating):

<script>
    const foo = 123;
    var bar = 456;
    let baz = 789;
</script>

<script type="module">
    console.log(foo); // logs "123"
    console.log(globalThis.foo); // logs "undefined"
    console.log(bar); // logs "456"
    console.log(globalThis.bar); // logs "456" (not undefined!)
    console.log(baz); // logs "789"
    console.log(globalThis.baz); // logs "undefined"
</script>

Only the var variables define a property on the global object.