Allow intersection type guards for multiple parameters

Search Terms

type guard multiple parameters

Suggestion

I'd like to write a type guard that takes two parameters and changes the type of both of them.

Use Cases

My specific use case is to try to make the following pattern (somewhat) more type-safe:

class Foo<TFeature, TOther> {
    // If featureCtor is null, TFeature will never be used.
    constructor(private readonly featureCtor: { new(): TFeature } | null) { }

    isFeature(thing: any): thing is TFeature {
        return !!this.featureCtor && thing instanceof this.featureCtor;
    }

    bar(thing: TFeature|TOther) {
        if (this.isFeature(thing)) {
            // Type guard should prove that this.featureCtor is not null
            new this.featureCtor();
        } else {
            // Type guard should prove this
            const x: TOther = thing;
        }
    }    
}

Examples

isFeature(thing: any, ctor: { new(): TFeature } | null): (thing is TFeature)&(ctor is { new(): TFeature }) {
    return !!this.featureCtor && thing instanceof this.featureCtor;
}

It would be even nicer to allow type guards to operate on readonly fields, so I wouldn't need to pass this.featureCtor as a parameter.

I also tried

constructor(private readonly featureCtor: TFeature extends never ? null : { new(): TFeature }) { }

But that didn't work.

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. new expression-level syntax)

まず、issue にある 2 パラメーターの type guard の例と、readonly フィールドを含む両方のパラメーターを絞り込むために要求されている動作を調べます。issue ではソースファイルもテストも指定されていません。生成される JavaScript や既存のランタイム動作を変更せずに intersection type guards をサポートできれば完了です。