readonly function arguments

[mohsen1]

Search Terms

readonly parameters, readonly arguments, read only, function

Suggestion

Allow specifying a readonly modifier to function and method arguments

function foo(mutableString: string, readonly immutableString: string) {
 mutableString = 'bar' // ok
 immutableString = 'bar' // not ok
}

With #29435 we can do this for rest arguments too:

function foo(readonly ...immutableStrings: string[]) {}
// the same as
function foo(...immutableStrings: readonly string[]) {}

Use Cases

There is a ESLint rule for not allowing overriding incoming function argument no-param-reassign

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.

[RyanCavanaugh]

Duplicate #18497