CODEWITHSUNDEEP
typescript
LppEdd
LppEdd

Reputation: 21114

TypeScript - type guard for parameter property

Let's say I have a function such as

export function is<T extends string>(
  element: ModdleElement,
  type: T
): element is BpmnElement<T>;

Let's add an overload

export function is<T extends string>(
  element: Base,
  type: T
): boolean;

The Base interface is

interface Base {
   businessObject: ModdleElement;
}

I'd like to apply the type guard also for the overload, to the businessObject field, so that

if (is(base, '...')) {
   const t = base.businessObject.myProp; // Correctly cast
}

Is this possible?

Upvotes: 0

Views: 432

Answers (1)

Titian Cernicova-Dragomir
Titian Cernicova-Dragomir

Reputation: 249506

Type guards usually work by narrowing unions. In this case Base is not a union, but we ca still get things to work using an intersection with something that has a property of the specified sub-type in the is return type . 

class ModdleElement { p: any; } // placeholder
class BpmnElement<T> extends ModdleElement { myProp: T } // placeholder

export declare function is<T extends string>(
    element: ModdleElement,
    type: T
): element is BpmnElement<T>;
export declare function is<T extends string>(
    element: Base,
    type: T
): element is (Base & { businessObject: BpmnElement<T> });


interface Base {
    businessObject: ModdleElement;
}
declare let base: Base;

if (is(base, '...')) {
    const t = base.businessObject.myProp; // Correctly cast
}

Upvotes: 2

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