In TypeScript, is there a shorthand for `index === undefined ? undefined : someArray[index]`?

Issue

Given the variables with types:

const someArray: number[];
const index: number | undefined;

is there a shorthand in TypeScript for the expression:

index === undefined ? undefined : someArray[index]

The above expression can become quite verbose especially if index itself is also an expression.

I understand the expression can be wrapped into a function, I'm just curious whether there's a builtin way to write this more concisely. For example, in JS, this can be simply: someArray[index].

Solution

The expression

index === undefined ? undefined : someArray[index]

is not equivalent to

someArray[index]

because if index === undefined, the former expression will definitely evaluate to undefined, while the latter is indexing into someArray with undefined as the property name, which coerced undefined to the string "undefined". Thus someArray[undefined] is equivalent to someArray.undefined. Will someArray.undefined exist? Probably not. So in practice this is likely to be undefined, but it is technically possible:

const myArray: number[] = Object.assign([123], { undefined: "xyz" });
const index: number | undefined = Math.random() < 0.5 ? 0 : undefined;
console.log(myArray[index] === undefined) // false

myArray[index] in the above example cannot be undefined.

---

Furthermore, even if they were equivalent, TypeScript adds a type system on top of JavaScript in order to add some type safety while still allowing for idiomatic JavaScript code. Indexing into an object or array with a key that isn't of type string or number or symbol is really not idiomatic JavaScript. TypeScript takes the opinionated position that it is probably a mistake to use undefined as an index, because it is rarely what people actually intend to do. So even if it turned out that there were no edge cases to someArray[index], it would still be considered illegal TypeScript. For more information, see What does "all legal JavaScript is legal TypeScript" mean?

If you really want to write someArray[index] you can do so via type assertion and/or the any type to loosen type checking:

someArray[index as any]

If not, then index === undefined ? undefined : someArray[index] or a function that abstracts the operation is probably your best bet.

Playground link to code

Answered By - jcalz