using generics in TypeScript

Question

I have created this class that works with generics in TypeScript

export class ModelTransformer {

    static hostelTransformer: HostelTransformer;

    static fromAtoB(instance: T): U {
        if (instance instanceof HostelType) {
            return ModelTransformer.hostelTransformer.fromAtoB (instancet);
        }
    }

}

But when I compile it I have those compilation errors:

src/model/transformer/model.transformer.ts:11:47 - error TS2304: Cannot find name 'T'.

11     static fromAtoB(instance: T): U {

src/model/transformer/model.transformer.ts:11:51 - error TS2304: Cannot find name 'U'.

11     static fromAtoB(instance: T): U {

I have tried the solution proposed, but then I have this error:

error TS2366: Function lacks ending return statement and return type does not include 'undefined'.

additionally to the Bauke's answer: you need to change the returning type of your method to U | void, for example, since it doesn't return anything in case instance is not instanceof HostelType. — Arfeo
– Arfeo, Commented Dec 19, 2019 at 8:32

user12251171 · Accepted Answer · 2019-12-18 15:45:02Z

1

You need to define a "type variable" as described in the handbook, like so:

static fromAtoB<T, U>(instance: T): U {
    if (instance instanceof HostelType) {
        return ModelTransformer.hostelTransformer.fromAtoB (instancet);
    }
}

answered Dec 18, 2019 at 15:45

user12251171

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1 Comment

kaya3 Over a year ago

This is the correct answer, but because U doesn't have a lower bound and isn't provided as a type argument to fromAtoB, then it's still going to have a type error unless fromAtoB's return type is any, never or another not-lower-bounded type parameter. (Not that Typescript currently supports lower bounds for type variables.)

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