I encountered an unexpected result when I executed the code const s: string = ['a'][1];. Instead of receiving a type error from the Typescript compiler, it returned undefined. This was surprising to me because I believed I was trying to assign an array to a string.
Could someone shed some light on how this expression is interpreted?
Here is a breakdown of your code snippet to help you understand it better:
const myArr: string[] = ['a'];
const s: string = myArr[1];
The code above is similar to the one you provided. Since ['a'] is already a string array (string[]), any index you access will be of type string.
If you were to modify your snippet like this:
const s: string = (['a'] as const)[1];
You would encounter some interesting type errors. TypeScript now interprets ['a'] as a tuple type: type T = ['a'], indicating it is a tuple of length 1. Therefore, accessing index 1 on the tuple results in an error. To learn more about tuples in TypeScript, you can visit: https://www.typescriptlang.org/docs/handbook/2/objects.html#tuple-types
Lastly, as suggested by @jcalz, enabling the noUncheckedIndexedAccess flag as a compiler option allows for potential undefined types when accessing any index on an array, providing a more precise type checking.
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