Type narrowing generics/unknown doesn't seem to work with typeof and null check

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🔎 Search Terms

typeof object, null checks, narrowing object, narrow generics

🕗 Version & Regression Information

All

This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

function func<T>(obj: T): boolean {
  if (obj && (typeof obj === 'object') && obj.hasOwnProperty('xyz')) {
    return true;
  }
  return false;
}

declare const testVariable: unknown;

if (testVariable !== null && (typeof testVariable === 'object') && testVariable.hasOwnProperty('xyz')) {
}

if ((typeof testVariable === 'object') && testVariable !== null && testVariable.hasOwnProperty('xyz')) {
}

🙁 Actual behavior

The generic function doesn't infer that obj must be an object of some sort.

The first if doesn't infer that testVariable can't be null.

🙂 Expected behavior

The generic function should infer that T is an object because we did a null check and checked typeof.

Both ifs should infer that testVariable can't be null.

Notes

Is this just me missing some 'known behaviour' typescript has? the two conditions seem to depend on ordering, maybe thats just a known thing i was unaware of (i.e. by design)?

Duplicate of #28131.

The second part of this is. There are two issues i would say, the generic function can be re-ordered to have the same order and will still error.

e.g.

function func<T>(obj: T): boolean {
  // Errors because `hasOwnProperty` doesn't exist on T, doesn't know it is an object
  if ((typeof obj === 'object') && obj !== null && obj.hasOwnProperty('xyz')) {
    return true;
  }
  return false;
}

T extends Object

Otherwise you can pass in this object: { hasOwnProperty: () => "example" }.

T doesn't extend object...

function func<T>(obj: T): boolean {
  if ((typeof obj === 'object') && obj !== null && obj.hasOwnProperty('xyz')) {
    return true;
  }
  return false;
}

func('foo'); // false
func(123); // false
func({xyz: 123}); // true
// etc etc

See my update. If you don't make T extends Object you can pass in an object like { hasOwnProperty: () => "example" }.

And 'foo' and 123 both extend Object.

function func<T extends Object>(_val: T) {}

// Works fine
func('example')
func(123)
func(true)

out of interest, why isn't that behaviour the same without generics?

unknown could also be { hasOwnProperty: () => "example" } could it not?

const foo: unknown = {hasOwnProperty: () => "blah"};

if (typeof foo === 'object' && foo !== null && foo.hasOwnProperty('blah')) {
  // WORKS
}

so why is it any different just because we have a T rather than unknown? which is just as lacking in type

i suppose because T means the compiler could have inferred the type from higher up the chain and would know hasOwnProperty is something different here, whereas with unknown it really has no clue so is a bit dumber about it. so only because the compiler knows more in the case of generics it disallows this

I don't know about the specifics, you'd need to wait for the TypeScript team or someone else more knowledgable.

But your guess sounds like a good one to me. It's similar to this example:

function func<T extends Object>(value: T) {
  if (isTest(value)) { value; }
}

type Test = { test: string }
function isTest(val: unknown): val is Test { return true }

value is not narrowed to Test, it's narrowed to T & Test. The generic type argument is kept.

There are some weird corner cases here I don't recall that lead to this. It's certainly not desirable.

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Type narrowing generics/unknown doesn't seem to work with typeof and null check #43997

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Type narrowing generics/unknown doesn't seem to work with typeof and null check #43997

Description

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Notes

Activity

MartinJohns commented on May 7, 2021

43081j commented on May 7, 2021

MartinJohns commented on May 7, 2021

43081j commented on May 7, 2021

MartinJohns commented on May 7, 2021

43081j commented on May 7, 2021

MartinJohns commented on May 7, 2021

RyanCavanaugh commented on May 11, 2021

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