TypeScript: reduce function - No overload matches this call

Question

Trying to write a basic reducer to return an array of (value of) a key from an array of objects. Some of the key can be missing or undefined.

My code:

const data = [{Key: 56}, {Key: undefined}, {}, {Key: 44}]

const keys = data.reduce((prev, curr) => {
  if (curr.Key) {
    return [...prev, curr.Key];
  } else return prev;
}, []);

It works fine as a plain JS but the TS compiler does not like it. Playground Link

How to fix this? mainly what am I doing wrong? I am a TS noob.

Error:

No overload matches this call.  

Overload 1 of 3, '(callbackfn: (previousValue: { Key: number; } | { Key: undefined; } | { Key?: undefined; }, currentValue: { Key: number; } | { Key: undefined; } | { Key?: undefined; }, currentIndex: number, array: ({ Key: number; } | { ...; } | { ...; })[]) => { ...; } | ... 1 more ... | { ...; }, initialValue: { ...; } | ... 1 more ... | { ...; }): { ...; } | ... 1 more ... | { ...; }', gave the following error.  
Argument of type '(prev: never[], curr: { Key: number; } | { Key: undefined; } | { Key?: undefined; }) => number[]' is not assignable to parameter of type '(previousValue: { Key: number; } | { Key: undefined; } | { Key?: undefined; }, currentValue: { Key: number; } | { Key: undefined; } | { Key?: undefined; }, currentIndex: number, array: ({ Key: number; } | { ...; } | { ...; })[]) => { ...; } | ... 1 more ... | { ...; }'.  
Types of parameters 'prev' and 'previousValue' are incompatible.  
Type '{ Key: number; } | { Key: undefined; } | { Key?: undefined; }' is not assignable to type 'never[]'.  
Type '{ Key: number; }' is missing the following properties from type 'never[]': length, pop, push, concat, and 26 more.  
Overload 2 of 3, '(callbackfn: (previousValue: never[], currentValue: { Key: number; } | { Key: undefined; } | { Key?: undefined; }, currentIndex: number, array: ({ Key: number; } | { Key: undefined; } | { Key?: undefined; })[]) => never[], initialValue: never[]): never[]', gave the following error.  
Argument of type '(prev: never[], curr: { Key: number; } | { Key: undefined; } | { Key?: undefined; }) => number[]' is not assignable to parameter of type '(previousValue: never[], currentValue: { Key: number; } | { Key: undefined; } | { Key?: undefined; }, currentIndex: number, array: ({ Key: number; } | { Key: undefined; } | { Key?: undefined; })[]) => never[]'.  
Type 'number[]' is not assignable to type 'never[]'.  
Type 'number' is not assignable to type 'never'.

Answer 1

Assign the type of the accumulator ([]) as number[] (TS playground):

const keys = data.reduce((prev, curr) => {
  if (curr.Key) {
    return [...prev, curr.Key];
  } else return prev;
}, [] as number[]);