Typescript extensions. Conditional Types

Question

I was trying to use conditional type but it does not work as expected. I was expecting type of abc would be number but it returns a string. Any help on the same would be appreciated.

class TableIdClass {
    public tableId?: string;

    constructor(props: TableIdClass) {
        const { tableId } = props;
        this.tableId = `${Math.random()}`;

    }
}

export class TableBase extends TableIdClass {

    public createDate?: Date;

    constructor(props: TableBase) {
        super(props)
        const { createDate } = props;
        this.createDate = (createDate) ? createDate : new Date();
    }
}

export class EntityBase extends TableBase {
    public entityId?: string;
    public entityName?: string;
    public isActive?: boolean;

    constructor(props: EntityBase) {
        super(props)
        const { entityId, entityName, isActive } = props;
        this.entityId = entityId;
        this.entityName = (entityName) ? entityName : '';
        this.isActive = (typeof isActive === 'undefined') ? true : isActive;
    }
};

class SomeClass extends TableIdClass {

    constructor(prop: SomeClass) {
        super(prop)

    }
}

type abc = SomeClass extends EntityBase ? string : number; // Returns string.

Nurbol Alpysbayev · Accepted Answer · 2018-10-17 08:59:48Z

2

You should add to EntityBase some non-optional property e.g.

class EntityBase {

    public isEntityBase = undefined
    ...
}

This is because TypeScript uses structural subtyping, in other words it checks whether an object implements some interface, by looking at the object's structure (names of properties).

In order to not pollute the API of EntityBase, you could use Symbols:

const isEntityBase = Symbol()

class EntityBase {

    public [isEntityBase] = undefined


}

edited Oct 17, 2018 at 8:59

answered Oct 17, 2018 at 8:18

Nurbol Alpysbayev

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5 Comments

Amol Gupta Over a year ago

nope does not help. checked it in typescript playground

Titian Cernicova-Dragomir Over a year ago

@NurbolAlpysbayev better yet, make it private (so no classes can mistakenly implement the interface), and don't assign a value to it, type it as undefined, no runtime cost :) private [isEntityBase]: undefined;

Nurbol Alpysbayev Over a year ago

Thank you, @TitianCernicova-Dragomir. You are true about true :-). However it's public because otherwise IDE will complain "unused private bla bla".

Amol Gupta Over a year ago

btw we could make it private and ignore it seems to be a fair place to use ts-ignore.

Titian Cernicova-Dragomir Over a year ago

@NurbolAlpysbayev forgot about unused privates :)

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