[feature] support union type rest parameter

[Meir017]

Search Terms

type, union, rest

Suggestion

allow rest operator on a union type when all cases are array

Use Cases

in order to declare a ctor with overloads, I wrote something like this

type TimeSpanCtorArguments = [number]
    | [number, number, number]
    | [number, number, number, number]
    | [number, number, number, number, number];

class TimeSpan {
    private readonly _ticks: number;
    constructor(...args: TimeSpanCtorArguments) {
        if (typeof args.length === 1) {
            [this._ticks] = args;
        } else if (args.length === 3) {
            const [hours, minutes, seconds] = args;
        } else if (args.length === 4) {
            const [days, hours, minutes, seconds] = args;
        } else {
            const [days, hours, minutes, seconds, milliseconds] = args;
        }
    }
}

I get the error: [ts] A rest parameter must be of an array type.

on the line

    constructor(...args: TimeSpanCtorArguments) {

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. new expression-level syntax)

[Meir017]

might be related to #15972

[Meir017]

@RyanCavanaugh any update on this?

[Andarist]

@RyanCavanaugh this already works since 3.3 so the issue can be closed:
TS 3.3 playground