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Suggestion: readonly constructor parameters #7590
Description
I've been using the new readonly properties feature (#6532) and have found it to be a very productive alternative to using getters or getWhatever() functions, or as is often the case, simply doing nothing to enforce read-only properties. However I'm really yearning for the ability to mark constructor parameters as readonly.
In 1.8 a common pattern for me is to write classes where all properties are public and readonly like so:
export class Foo {
constructor(public bar: number,
public baz: number) {}
}But now with readonly I'm missing out unless I rewrite the above code to:
export class Foo {
public readonly bar: number;
public readonly baz: number;
constructor(bar: number, baz: number) {
this.bar = bar;
this.baz = baz;
}
}Which is pretty cumbersome. What I'd really like to be able to write is:
export class Foo {
constructor(public readonly bar: number,
public readonly baz: number) {}
}Right now I'm worried about hitting 2.0 and having to rewrite all my constructors just to take advantage of readonly. Because it's an advantage I definitely want to have.
Activity
DanielRosenwasser commentedon Mar 19, 2016
So would a
readonlyparameter be treated as aconstwithin the body of its constructor?jahewson commentedon Mar 21, 2016
No, it would simply be a shorthand for creating properties in the manner of
constructor(public foo) { ... }, only we'd get areadonlyproperty.leonyu commentedon Apr 4, 2016
This can probably be implemented using a decorator on the function doing a defensive copy on the arguments. However, that would be a runtime check.
jahewson commentedon Apr 5, 2016
Leon Yu (@leonyu) Copying the arguments is not what I'm after. I'm asking for exactly the same semantics as
readonly. No copying, no runtime checks. Just a shorthand for creating areadonlyproperty, in the same way that addingpublicto a parameter is a shorthand way to create a public property.nbransby commentedon May 4, 2016
+1
RyanCavanaugh commentedon May 9, 2016
Makes a lot of sense. We'll include this along with the first release with
readonlyjahewson commentedon May 10, 2016
Awesome, thanks!
Fix #7590: Allow 'readonly' to be used in constructor parameters
Fix #7590: Allow 'readonly' to be used in constructor parameters
Merge pull request #8555 from Microsoft/readonly_ctr
aluanhaddad commentedon May 23, 2016
Really glad this is implemented. In addition to the increased expressiveness, this allows a fair amount of code currently using accessibility modifiers for the sake of brevity to be written with clearer intent.