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Incorrect type inference when using object index #17011
Description
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
Activity
zlepper commentedon Jul 7, 2017
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 commentedon Jul 7, 2017
Rasmus Hansen (@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 commentedon Jul 7, 2017
Andy (Andrewkraft) (@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 wherexis a variable since we'd also have to track that there are no intervening assignments tox. I suggest changing the code to:This behaves as expected and is also more efficient.
zlepper commentedon Jul 7, 2017
That is also what i wrote in the OP
I just wanted to make sure it was intended. Thank you!
And thanks for the Great language :)
ahejlsberg commentedon Jul 7, 2017
Rasmus Hansen (@zlepper) Right, I missed that. And you're welcome!