Incorrect type inference when using object index

[zlepper]

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)
2.4.1

Code

function test(paramsObject: { [key: string]: string | Array<string> }) {
    for (let key in paramsObject) {
        if (paramsObject.hasOwnProperty(key)) {
            if (Array.isArray(paramsObject[key])) {
                doArray(paramsObject[key]);
            } else {
                doString(paramsObject[key]);
            }
        }
    }
}


function doString(s: string) {
    console.log(s);
}

function doArray<T>(a: Array<T>) {
    console.log(a);
}

Expected behavior:
Typescript should be able to infer that paramsObject[key] is still a string or string array after Array.isArray has been called. It can't change.
It works correct if I assign paramsObject[key] to a variable beforehand, and then use that variable in both the check and the doArray and doString.

Actual behavior:
Argument of type 'string | string[]' is not assignable to parameter of type 'string[]' for doArray
and
Argument of type 'string | string[]' is not assignable to parameter of type 'string'. for doString

A minimal example:

function f(x: { y: string | number }): string {
    if (typeof x["y"] === "string") {
        return x["y"];
    }
    return "";
}

Works with x.y.

[zlepper]

I made the example in the OP in the playground, so i'm pretty sure typescript complains, and I have seen it in my local code, that's why i noticed in the first place.

[kitsonk]

@zlepper Andy reduced your example to a more minimum reproducible which exhibits the behaviour. In other words, he is suspecting it is an issue.

[ahejlsberg]

@Andy-MS Your example is a duplicate of #10530, but isn't the same as the OP.

The OP example is working as intended. In the example the index expresssion is a variable and not a string literal. I don't think it is likely we'll ever do control flow based narrowing of obj[x] expressions where x is a variable since we'd also have to track that there are no intervening assignments to x. I suggest changing the code to:

function test(paramsObject: { [key: string]: string | Array<string> }) {
    for (let key in paramsObject) {
        if (paramsObject.hasOwnProperty(key)) {
            const value = paramsObject[key];
            if (Array.isArray(value) {
                doArray(value);
            } else {
                doString(value);
            }
        }
    }
}

This behaves as expected and is also more efficient.

[zlepper]

That is also what i wrote in the OP

It works correct if I assign paramsObject[key] to a variable beforehand

I just wanted to make sure it was intended. Thank you!

And thanks for the Great language :)

[ahejlsberg]

@zlepper Right, I missed that. And you're welcome!