Typescript property type guards on unknown

Question

I'm trying to type guard an unknown type

const foo  = (obj: unknown) => {
    if (typeof obj === 'object' && obj) {
        if ('foo' in obj && typeof obj.foo === 'string') {
            return obj.foo;
        }
    }
};

But I'm getting

Property 'foo' does not exist on type 'object'.

I also tried with is expression does not work:

const foo  = (obj: unknown): obj is { foo: 'string' } => {
    if (typeof obj === 'object' && obj) {
        if ('foo' in obj && typeof obj.foo === 'string') {
            return obj;
        }
    }
    throw new Error();
};

Answer 1

Please consider using this helper:

const hasProperty = <Obj, Prop extends string>(obj: Obj, prop: Prop)
  : obj is Obj & Record<Prop, unknown> =>
  Object.prototype.hasOwnProperty.call(obj, prop);

in your case. in operator works as expected mostly with unions. Please see here , here and here

Working solution:

const hasProperty = <Obj, Prop extends string>(obj: Obj, prop: Prop)
  : obj is Obj & Record<Prop, unknown> =>
  Object.prototype.hasOwnProperty.call(obj, prop);

const foo = (obj: unknown) => {
  if (typeof obj === 'object' && obj) {
    if (hasProperty(obj, 'foo') && typeof obj.foo === 'string') {
      return obj.foo;
    }
  }
};

Playground

However, since you want to throw an error if obj is invalid, you can use assert function :

const hasProperty = <Obj, Prop extends string>(obj: Obj, prop: Prop)
  : obj is Obj & Record<Prop, unknown> =>
  Object.prototype.hasOwnProperty.call(obj, prop);

function foo(obj: unknown): asserts obj is { foo: string } {
  const isValid =
    typeof obj === 'object' &&
    obj &&
    hasProperty(obj, 'foo') &&
    typeof obj.foo === 'string';

  if (!isValid) {
    throw new Error();
  }

};

declare var obj: unknown;

foo(obj);

obj.foo // ok

Playground

Answer 2

You're going to have to give TypeScript a little help here:

type fooObj = object & { foo: unknown };
const foo = (obj: unknown) => {
    if (typeof obj === 'object' && obj) {
        if ('foo' in obj && typeof (obj as fooObj).foo === 'string') {
            return (obj as fooObj).foo;
        }
    }
};