Add `--strictObjectIterables` (bikeshed): exclude primitive (`string`) from `Iterable` types

🔍 Search Terms

"iterable object", "string"

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

At the TC39 meeting in 2024-07, it was decided that Iterable expects objects.

Reject primitives in iterable-taking positions

Any time an iterable or async-iterable value (a value that has a Symbol.iterator or Symbol.asyncIterator method) is expected, primitives should be treated as if they were not iterable. Usually, this will mean throwing a TypeError. If the user provides a primitive wrapper Object such as a String Object, however, it should be treated like any other Object.

Although primitive Strings are default iterable (String.prototype has a Symbol.iterator method which enumerates code points), it is now considered a mistake to iterate a String without specifying whether the String is providing an abstraction over code units, code points, grapheme clusters, or something else.

NB: This convention is new as of 2024, and most earlier parts of the language do not follow it. In particular, positional destructuring (both binding and assignment), array spread, argument spread, for-of loops, yield *, the Set and AggregateError constructors, Object.groupBy, Map.groupBy, Promise.all, Promise.allSettled, Promise.any, Promise.race, Array.from, the static from methods on typed array constructors, and Iterator.from (Stage 3 at time of writing) all accept primitives where iterables are expected.

https://github.com/tc39/how-we-work/blob/main/normative-conventions.md#reject-primitives-in-iterable-taking-positions

To follow this decision, the Iterable and string types should be separated, and APIs that accept both types should be required to explicitly specify Iterable<any> | string. Since this would be a breaking change, how about adding a new --strictObjectIterables (bikeshed) option?

In practice, the upcoming ReadableStream.from will accept Iterable<any> and AsyncIterable<any>, but will be restricted to objects. https://github.com/whatwg/streams/pull/1310

📃 Motivating Example

Enabling the --strictObjectIterables option raises a type error in the following example:

const iterable: Iterable<any> = "string";

💻 Use Cases

1. What do you want to use this for?

   Used for ReadableStream.from and other APIs to be added in the future.

2. What shortcomings exist with current approaches?

   Iterable object and string are not distinguished by default.

3. What workarounds are you using in the meantime?

   Maybe Iterable<any> & object can reject string.

Start by reviewing the proposed --strictObjectIterables behavior and the linked TC39 convention, then trace how Iterable and string are represented and checked in TypeScript. Use the motivating assignment as a type-checking case and determine the affected APIs. Done means the option rejects primitive strings as Iterable while allowing explicit Iterable | string usage.