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TypeScript doesn't assume the value type in a ternary operator when it has been assigned before if the value is an object key #50356
Description
Bug Report
🔎 Search Terms
ternary operator, props object, object is possibly null
🕗 Version & Regression Information
- This happens in version 4.7.4 and 4.8.0-beta
⏯ Playground Link
Playground link with relevant code
💻 Code
type Props = {
value: string | Array<string> | [] | null;
};
function example1(props: Props) {
const isValueAnArray = Array.isArray(props.value);
const isValueEmpty = !(isValueAnArray ? !!props.value.length : !!props.value);
^ (property) value: string | string[] | [] | null
Object is possibly 'null'.
}
function example2(props: Props) {
const isValueAnArray = Array.isArray(props.value);
const isValueEmpty = !(Array.isArray(props.value) ? !!props.value.length : !!props.value); // No errors
}
function example3(value: string | Array<string> | [] | null) {
const isValueAnArray = Array.isArray(value);
const isValueEmpty = !(Array.isArray(value) ? !!value.length : !!value); // No errors
}🙁 Actual behavior
example1 throws an error because TS thinks props.value might be null when checking its length.
🙂 Expected behavior
There should be no error, like in the other two functions.
Activity
whzx5byb commentedon Aug 18, 2022
From #44730:
So you have to mark the property
valuereadonly to make it work:type Props = { - value: string | Array<string> | [] | null; + readonly value: string | Array<string> | [] | null; };