Proposal: ArrayValue type

[avaly]

Suggestion

🔍 Search Terms

array type
array value
array value type

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

An utility type to obtain an array's items type.

type Value = ArrayValue<T[]>; // T

📃 Motivating Example

TS 4.5 introduced the Awaited utility type for promises. This proposals adds an utility type with similar syntax for arrays.

💻 Use Cases

When building components for a list UI and need to use some API generated types (e.g. GraphQL generated types):

type Item = ArrayValue<GeneratedResponse['nodes']>;

Current workaround: using Foo[number], but it's less readable.

[MartinJohns]

See #39522 (comment):

We've opted to not include utility type aliases in the lib unless they're required for declaration emit purposes.

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Personally I don't see much value, when it's trivially done using T[number] already.

type ArrayType<T extends any[]> = T[number]
type Foo = ArrayType<number[]>
//   ^?