Provide a ReadonlyRegExp type

Search Terms

readonlyregexp readonly regexp

Suggestion

Provide a new ReadonlyRegExp type that represents a read only RegExp.

Use Cases

This is similar to other readonly types, e.g. ReadonlyArray -- I prefer my code to be immutable by default. I have a DeepReadonly helper that could map RegExp to ReadonlyRegExp.

Note that Readonly<RegExp> does not work:

'a'.replace(/a/ as Readonly<RegExp>, 'b')

/*
No overload matches this call.
  The last overload gave the following error.
    Argument of type 'Readonly<RegExp>' is not assignable to parameter of type 'string | RegExp'.
      Type 'Readonly<RegExp>' is missing the following properties from type 'RegExp': [Symbol.match], [Symbol.replace], [Symbol.search], [Symbol.split](2769)
*/

Examples

const emailRegExp = /[a-z]@[a-z\.]/ as ReadonlyRegExp;

export function suppressPII(message: string) {
    return message.replace(emailRegExp, '');
}

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.

The issue names no implementation files, tests, or entry points. Start by tracing how TypeScript declares RegExp and existing readonly types, then determine the declaration and compatibility changes needed for the shown replace example. Done means a ReadonlyRegExp type is available and usable where the example expects a RegExp.