Anticipated result would be number | never[].
However, it is actually number | never[] | undefined.
type T1 = (number | undefined)[];
const myFunc = (arr: T1, index: number): any => {
const result = arr[index] === undefined ? [] : arr[index];
return result; // number | never[] | undefined
}
What causes this behavior to occur?
Flow analysis in TypeScript helps narrow down variable types, but has limitations when it comes to narrowing down arrays with undefined elements:
type T1 = (number | undefined)[]
const myFunc = (arr: T1, index: number) => {
if (arr[index] === undefined) {
return []; // never[]
}
//the type of `arr` remains as T1
let temp = arr[index]; //number | undefined
return temp;
}
It can be challenging to represent the type of arr after the if block – should it still be considered as an array of number|undefined, except for that particular dynamic index? Falling back to T1 may be the simplest solution.
A ternary operator may not improve upon this situation significantly.
In contrast, TypeScript shines in narrowing down types for a result variable within flow control statements:
type T1 = (number | undefined)[]
const myFunc = (arr: T1, index: number) => {
const result = arr[index];
if (result === undefined) {
return []; // never[]
}
//here, the type of `result` is correctly determined as number
return result;
}
While variables like arr and result can have their types narrowed down by control flow, expressions like arr[index] will depend on the types of arr and index at that specific point.
It appears that TypeScript may require assistance in correctly inferring types based on your ternary expression. There are two ways you can resolve this issue:
Option 1: Introduce a variable
const myFunction = (data: T1, idx: number) => {
const item = data[idx];
const result = item === undefined ? [] : item;
return result // number | never[]
}
Option 2: Utilize the non-null assertion operator to clarify to the compiler that the value will always have a defined value:
const myFunction = (data: T1, idx: number) => {
const result = data[idx] === undefined ? [] : data[idx]!
return result // number | never[]
}
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