Consider this scenario:
type Example = {
x: string,
y: number
}
const a: Example = {
x: "x",
y: 1
}
const b: Example = {
x: "y",
y: 2
}
const issue = (keys: (keyof Example)[]) => {
keys.forEach(key => {
a[key] = b[key]
})
}
An error with code TS2322 has been encountered and it's proving difficult to troubleshoot. (or to explain in more detail).
Here is the complete error message:
Type 'string | number' is not assignable to type 'never'.The issue lies in how keyof dummy specifies 'a' | 'b', without properly distinguishing between them. TypeScript lacks the intelligence to understand that if a[key] represents a string, then b[key] will also represent a property containing a string. (It currently interprets b[key] as string | number, which cannot be assigned to a value that is strictly either a string or a number, but not both.)
To address this, you must utilize generics to correctly type the key and allow for the assignment:
const problem = <T extends keyof Dummy>(key: T) => {
a[key] = b[key];
}
If dealing with an array of keys instead, it may be beneficial to use Object.fromEntries to form a partial object beforehand, followed by employing Object.assign:
const problem = (keys: (keyof Dummy)[]) => {
const partial = Object.fromEntries(
keys.map(key => [key, b[key]])
);
Object.assign(a, partial);
}
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