Add missing `toString` declarations for base types that have them

Search Terms

toString, RegExp, Regular Expression, Boolean, Error

Suggestion

In @typescript-eslint, we have a rule no-base-to-string. This rule ensures that any value that's being coerced to a string has a toString method defined on it.
This is to help catch cases where you accidentally coerce something to a string, and end up with an [object Object] in your strings.

This rule has a pretty simple implementation - it just gets the declarations of the toString method on the type, and ensures that none of the declarations belong to the Object type.
The logic relies upon the fact that every type that actually has a toString method explicitly declares a toString method.

A few of our users ran into cases that were reporting errors for them, even though they shouldn't (https://github.com/typescript-eslint/typescript-eslint/issues/1655).
There's one that specifically has caused people some issues: Boolean/boolean.

I eyeballed the docs and the types, and I am pretty certain there are only 3 types that are missing a toString declaration:

• RegExp: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/toString
• Boolean: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean/toString
• Error: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/toString

Related

PR: #37839

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by locating the standard library declarations for RegExp, Boolean, and Error and compare their existing members with the corresponding JavaScript built-ins. Confirm that each missing toString declaration is represented in the types and verify the change with the relevant TypeScript test suite; done means all three types explicitly declare the method without changing emitted JavaScript.