`for..in` object type keys

[blaumeise20]

Bug Report

🔎 Search Terms

for..in object

🕗 Version & Regression Information

• I think it always was like that

⏯ Playground Link

Playground link with relevant code

💻 Code

const obj = {
    key1: "hello",
    key2: 124,
};

for (const key in obj) {
    console.log(obj[key]);
}

🙁 Actual behavior

There is an error "Element is implicitly 'any' because ... cannot be indexed with type 'string'".
key is of type string.

🙂 Expected behavior

It should work, and key should be "key1" | "key2".

[MartinJohns]

Objects are not sealed. Your expectation would be unsound.

[blaumeise20]

Hm I don't get what you mean. You can't add new properties like obj.abc = true, can you?

If that is possible, why not use keyof Whatever as the type? Because I can guarantee that obj will have a key as the value in the key variable.

[MartinJohns]

See #44706.

[fatcerberus]

Just for posterity:

interface FooPlus { foo: number, bar: string, qux: string }
interface Foo { foo: number, bar: string };

const foobar: FooPlus = { foo: 42, bar: "foo", qux: "moo" };
const foo: Foo = foobar;

for (const key in foo) {
    console.log(obj[key]);
}

key shouldn't be type "foo" | "bar" here because it will also take the value "qux".

[blaumeise20]

Oh, the problem is the edge case with more keys! I understand. But, is something like this possible without casts or ts-ignore? Because this is perfectly valid JS:

const defaultConfig = { someprop: true, abc: 123 };
const currentConfig = { someprop: true, abc: 456 };

const newConfig = Object.assign({}, currentConfig);
for (const key in newConfig) {
    if (newConfig[key] === defaultConfig[key])
        delete newConfig[key];
}

[MartinJohns]

Having more properties is hardly an edge case. Using a type assertion is an appropriate solution here.

[fatcerberus]

My experience has been, iterating over keys is generally paired with a string index signature/Record<string, T>, in which case the typing of the key as string isn’t an issue. If you know the shape of your object in advance enough to name the keys in the source upfront, you don’t typically need to iterate over it programmatically.

[blaumeise20]

That is true, but I want to be able to dynamically add properties to defaultConfig. I also created a type type Config = typeof defaultConfig, so I just have to add a new property to the default, and I'm done.

You think I should write this?

if (newConfig.someprop == defaultConfig.someprop) delete newConfig.someprop;
if (newConfig.abc == defaultConfig.abc) delete newConfig.abc;

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.