Rule proposal: Prevent / Warns if the code attempts to interpolate functions directly into string template

[chenxinyanc]

Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
• My proposal is not a "formatting rule"; meaning it does not just enforce how code is formatted (whitespace, brace placement, etc).
• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

This rule prevents potential erroneous implicit Function to string conversion in string interpolation expressions. Converting a function (or class itself) into string will cause function body being inserted into the string template, which could be undesirable in most of the cases.

I think we can leverage TypeScript typing information for the "Function" type checking. We don't need to cover the case where the interpolated value does not have Function type at compile-time.

Fail Cases

const formatErrorMessage = (error: unknown) => String(error);
// `formatErrorMessage` is of function type. Converting it into string results in function body being printed.
const message = "Error is {formatErrorMessage}";

Pass Cases

const formatErrorMessage = (error: unknown) => String(Error);
// Call the function to return the string result.
const message1 = "Error is {formatErrorMessage(err)}.";
// Or if you are converting function into string by design.
const message2 = "Error is {String(formatErrorMessage)}.";

Additional Info

No response

[bradzacher]

Try our rule restrict-template-expressions
https://typescript-eslint.io/rules/restrict-template-expressions

it does exactly this - and more!

playground