Allow more constructs to work as type guards for `unknown`

Search Terms

unknown type guard Related: https://github.com/Microsoft/TypeScript/pull/24439#issuecomment-394185089, https://github.com/Microsoft/TypeScript/issues/25172

Suggestion

Currently, only a very limited set of type guards are able to narrow the new unknown type:

• Array.isArray (because it is a typeguard for arg is any[]) and probably some more in the lib files
• instanceof
• self-written typeguards

However to make working with unknown types less awkward, I’d like to see a couple of other constructs being able to narrow the unknown type:

let x: unknown;
// Direct equality should narrow to the type we compare to
x === "1"; // should narrow x to string or the literal "1" type, similar for other types aswell

// All these should narrow x to {prop: any}
"prop" in x;
x.prop != null;
x.prop !== undefined;
typeof x.prop !== "undefined";

// typeof should work on properties of the unknown variable
typeof x.prop === "string"; // should narrow x to {prop: string}

Use Cases

Make unknown easier to work with!

// before, very verbose!
const x: unknown = undefined!;

function hasProp1(x: any): x is {prop1: any} {
	return "prop1" in x;
}
function hasProp2(x: any): x is {prop2: any} {
	return "prop2" in x;
}
// imagine doing this for many more properties

if (hasProp1(x)) {
	x.prop1;
	if (hasProp2(x)) {
		x.prop2;
	}
}

// ===========

// after, much more concise and less overhead
const x: unknown = undefined!;
if ("prop1" in x) {
	x.prop1;
	if ("prop2" in x) {
		x.prop2;
	}
}

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript / JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. new expression-level syntax)

[w0rp]

I would like it if type guards with unknown worked a little more like this.

let x: unknown

if (typeof x === 'object' && x !== null && 'foo' in x && typeof x.foo === 'string') {
  /* x is promoted to {foo: string} here */
}

I think the type promotion ought to work like so, if at all possible.

1. typeof unknown === 'object' -> object | null
2. (object | null) !== null -> object
3. 'foo' in object -> {foo: unknown}
4. typeof {foo: unknown}.foo === 'string' -> {foo: string}

[butchler]

Until this is fixed, this is a helper that can be used to make it easier to write manual type guards for unknown types that you expect to be nested objects:

export function isUnknownObject(x: unknown): x is { [key in PropertyKey]: unknown } {
  return x !== null && typeof x === 'object';
}

Example usage:

function example(x: unknown) {
    if (isUnknownObject(x) && isUnknownObject(x.prop) && typeof x.prop.subProp === 'string') {
        console.log(x.prop.subProp);
    } else {
        console.log('Could not find subProp');
    }
}

example({
    prop: {
        subProp: 'test',
    }
});
example({});

For more complicated use cases, using something like https://github.com/gcanti/io-ts is probably a better option than writing the type checks manually, but isUnknownObject can be useful for simple cases.