Add isObject

[fregante]

Type guards are hard:

interface Message {
	text: string;
}
function isMessage(value: unknown): value is Message {
  return value !== null && typeof value === 'object' && typeof value.type === 'string';
}

On the surface this is pretty straightforward and how you'd normally do type guards in any language, but:

error TS2531: Object is possibly 'null'.

19   return value !== null && typeof value === 'object' && typeof value.type === 'string';
                                                                  ~~~~~

TS2339: Property 'type' does not exist on type 'object'.

19   return value !== null && typeof value === 'object' && typeof value.type === 'string';
                                                                        ~~~~

Fun facts:

1. Moving the null check after the 'object' type check, fixes the first error, even if that shouldn't change anything.
2. If you use an intermediary Record<string, unknown> type, you can access that type without errors

interface Message {
	text: string;
}

function isObject(value: unknown): value is Record<string, unknown> {
  return typeof value === 'object' && value !== null;
}

function isMessage(value: unknown): value is Message {
  return isObject(value) && typeof value["type"] === 'string';
}

I think this isObject type guard is necessary in every type guard until TypeScript decides that object is actually Record<string, unknown> | null like one would expect.

Related:

• Proposal: PlainObject #107

[sindresorhus]

What type are you proposing? It sounds like you want: https://github.com/sindresorhus/is#objectvalue

Also, it seems like this will be fixed soon: microsoft/TypeScript#28131 (comment)

[fregante]

D'oh, you may be right. I'm not proposing a plain type.

However that PR only fixes the check order part, but it doesn't result into a Record type. The result is still an object without properties.