Flow: Exact object types shouldn't allow array access without an indexer annotation

Created on 27 Jun 2017 · 5Comments · Source: facebook/flow

Flow passes the following cleanly, but it shouldn't:

type T = {||};
declare var k: T;
k[1] = 7;

object model

Source

popham

👍5

Most helpful comment

I'd expect even a _non_-exact object type to fail with the "access of computed property/element . Indexable signature not found" error. The following should be a type error:

type Foo = { n: number };
const foo: Foo = {n: 0};
const name: string = 'n';
const prop: boolean = foo[name];

I want computed property access to be allowed only when there's an indexer property.

jfirebaugh on 12 Aug 2017

👍2 ❤1

All 5 comments

I'd expect even a _non_-exact object type to fail with the "access of computed property/element . Indexable signature not found" error. The following should be a type error:

type Foo = { n: number };
const foo: Foo = {n: 0};
const name: string = 'n';
const prop: boolean = foo[name];

I want computed property access to be allowed only when there's an indexer property.

jfirebaugh on 12 Aug 2017

👍2 ❤1

@jfirebaugh if you remove the string annotation from name it will give you your flow error.

type Foo = { n: number };
const foo: Foo = {n: 0};
const name = 'n';
const prop: boolean = foo[name];

jcready on 12 Aug 2017

Interesting -- I suppose in that case it gets typed as a literal, so essentially equivalent to foo['n'] which is also an error.

In the context where I encountered the issue, that's not an option however; name is not assigned from a string literal, and not explicitly annotated -- flow infers a string type.

jfirebaugh on 12 Aug 2017

Well, I believe this is the source of your problem. You can again work around this by changing your type definition: