Specialized error message when asking for a named object property on a tuple?

[JoshuaKGoldberg]

🔍 Search Terms

named object property tuple type lookup

✅ 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

Developers sometimes accidentally use [] to declare a type instead of {}. Later, when they try to access a property by string name, the error message doesn't indicate that it's a [] tuple instead of {} object. Can we get a specific error message for that example?

📃 Motivating Example

The following Label type alias seems to describe a reasonable tuple at first. But later on we see that the developer might have meant to describe an object type.

type Label = [
  text: string;
];

declare const label: Label;

label.text;
// Property 'text' does not exist on type 'Label'.

How about a specialized diagnostic indicating 'Label' is a tuple?

Property 'text' does not exist on type 'Label'.
  'Label' is a tuple type with numeric keys, not an object type with string keys.

...bikeshedding needed on that message 🤔

💻 Use Cases

• When a developer meant to write an object type but used a tuple instead
• When a developer misread a tuple type as an object type
• When a developer is new to tuples and/or types, and confused the two's syntaxes

Example real-world developer pain point (posted with permission): https://twitter.com/yagiznizipli/status/1736762337716039967

[RyanCavanaugh]

This seems like a good idea. In theory I think we can go further and offer the index in the error message, if it exists (maybe we should only do it in that case?)

[fatcerberus]

So that would be something on the order of

'Label' is a tuple type. Did you mean 'label[0]`?

Downside: This suggestion might be misleading, if the actual mistake the developer made was using [] instead of {} in the definition, as hinted in the Use Cases section of the OP.

[DanielRosenwasser]

Suggested error messages:

This value has no property named '{0}', but a corresponding label exists. Access its value at index '{1}'.

Related info on the label itself:

The label '{0}' does not name a property, it only documents the contents at a position.