Can typescript automatically create a typing that includes all the values from the items array's symbol property, instead of defining it manually like this:
type Item = { id: number; symbol: string; }
const items: Item[] = [
{ id: 1, symbol: "a"},
{ id: 2, symbol: "b" },
];
type Symbols = 'a' | 'b';
If you have a type annotation on the const items declaration in an actual array, you cannot determine the type from the array itself. The type annotation takes precedence over inferring the type from the actual value. However, removing the type annotation allows you to use typeof items to ascertain its type. You can then utilize indexed access to obtain the type of the symbol property.
It is important to note that using as const is necessary to maintain string literal types; otherwise, they will default to just string.
const items = [
{ id: 1, symbol: "a" },
{ id: 2, symbol: "b" },
] as const;
// Output: "a" | "b"
type Symbols = typeof items[number]['symbol']
By following this approach, you may no longer validate if the array elements are truly of type Item. To address this, refer to the technique outlined in this alternative answer. I opted for the type parameter S extends string instead of T extends Item to eliminate the need for as const.
function checkItems<S extends string>(arr: (Item & {symbol: S})[]) {
return arr;
}
const items = checkItems([
{ id: 1, symbol: "a" },
{ id: 2, symbol: "b" },
]);
Explore and experiment further with the code using the TypeScript Playground here.
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