If the array is empty, TypeScript does not perform type checking
Here is my code snippet:
type List = { name: string }[]
const l: List = []
// error
l[0].name
Is there a method to enable TS type checking in this scenario? How can I make TS type checking function properly?
Make sure to activate the noUncheckedIndexedAccess setting within the compilerOptions section of your tsconfig.json.
Once enabled, you will begin receiving TypeScript errors such as
Object is possibly 'undefined'.(2532)type List = { name: string }[]
const l: List = []
l[0].name // <-- error
l[0]?.name // <-- no error (ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining)
It's important to note that the noUncheckedIndexedAccess option does not verify the length of your array; it serves as a reminder that the index being accessed may not exist.
If your array (and its elements) are intended to be immutable, you can also utilize const assertion:
const l = [{ name: 'foo' }] as const
l[0].name // no error
l[1] // error: Tuple type 'readonly [{ readonly name: "foo"; }]' of length '1' has no element at index '1'.(2493)
If you simply wish to have a fixed-length array with mutable elements in TypeScript 4.1 and above, you can achieve this by:
// based on - https://stackoverflow.com/a/52490977
type _TupleOf<T, N extends number, R extends unknown[]> = R['length'] extends N ? Readonly<R> : _TupleOf<T, N, [T, ...R]>
type Tuple<T, N extends number> = N extends N ? (number extends N ? T[] : _TupleOf<T, N, []>) : never
type List = Tuple<{ name: string }, 1>
const l: List = [{ name: 'foo' }]
l[0].name // no error
l[1] // error: Tuple type 'readonly [{ name: string; }]' of length '1' has no element at index '1'.(2493)
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