What is the best way to create a data type that enables unrestricted index retrieval while ensuring that the value retrieved will never be

By enabling the noUncheckedIndexedAccess option in TypeScript, we ensure that when accessing object properties with arbitrary keys, the value type includes both the specified type and undefined.

This behavior is generally appropriate as it aligns with runtime expectations. However, I am interested in specifying that no property should ever return undefined, thus guaranteeing safe property access.

Here's an example code snippet:

type ArbitraryStringIndexAccess = {
  [key: string]: number
}

const object: ArbitraryStringIndexAccess = {}

const value: number = object.property
// Type 'number | undefined' is not assignable to type 'number'.
// Type 'undefined' is not assignable to type 'number'.
// (2322)

I have attempted to find possible solutions on my own without success. I would appreciate any insights on achieving this goal while maintaining the flag.

Edit:
There seems to be an existing issue related to this on GitHub, although it has not received significant attention:

noUncheckedIndexAccess - disable per type or property (use case: dynamic Proxy which returns value for every prop)

typescript

Answer №1

At this moment, TypeScript does not support this functionality. When you enable the `--noUncheckedIndexedAccess` compiler option, it will include `undefined` in the read type of index signatures. This is intentional. While there could potentially be a need to disable this feature for specific types (such as mimicking the behavior of a `Proxy` where the `get` handler always returns a value), there has not been enough community demand to implement such functionality yet. There is a feature request on microsoft/TypeScript#47594 asking for this support, but it is currently marked as "Awaiting More Feedback," indicating that more input from the community is needed before serious consideration is given.

In the meantime, you have a few options:

  • Disable the flag and manually add `undefined` where necessary
  • Use the flag and employ non-null assertions (`!`) whenever you access data from the Proxy
  • Rethink your approach and move away from using a `Proxy`, opting instead for an object with a `get()` method like `{ get(key: string): number }`, which aligns better with the type system and may be more user-friendly since most individuals do not anticipate objects having properties at random keys.

Answer №2

I wish for a way to guarantee that undefined is inaccessible in order to ensure safe property access.

Is this knowledge based on a rare object capable of being indexed by all strings? It seems unlikely, given the infinite number of strings and finite memory in computers, making it beyond TypeScript's capabilities at the moment.

Alternatively, are you certain because you only use specific strings for indexing? In that case, adjust the index signature accordingly:

type Outcome = "ok" | "fail";
const colorFor: {[O in Outcome]: string} = {
  ok: 'green',
  fail: 'red',
}

let outcome: Outcome = 'ok';
const color: string = colorFor[outcome]; // no chance of undefined as every outcome has a mapping

If your knowledge exceeds what the compiler can grasp, resort to a non-null assertion:

const color: string = o[k]!; // declare undefined as impossible

Be cautious though, as an incorrect assumption here could lead to color being undefined during runtime.

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