The group of typed elements cannot be assigned to an 'any[]' type array

I am trying to create an object with specific type properties. Inside this object, there is an array that should only contain elements of a certain type. I want to initialize the array as empty and push elements into it later on. However, I am encountering an error that I cannot seem to resolve. The code snippet from my app.ts is as follows:

const publishObject: MqttPublishObject = {
                        IsTurnedOff: detail.isTurnedOff,
                        processType: detail.processType.name === "linear" ? 0 : 1,
                        anchorPoints: <MqttPublishAnchorPoint>[]
                      };

But I am getting the following errors:

src/app/app.ts(51,25): error TS2322: Type 'MqttPublishAnchorPoint' is not assignable to type 'any[]'.
src/app/app.ts(51,39): error TS2352: Conversion of type 'undefined[]' to type 'MqttPublishAnchorPoint' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.

The type definitions are located in the respective files:

mqttPublishObject.ts:

export interface MqttPublishObject {
   IsTurnedOff : boolean;
   processType : number;
   anchorPoints : MqttPublishAnchorPoint[];
}

mqttPublishAnchorPoint.ts

export interface MqttPublishAnchorPoint {
    hour: number;
    minute: number;
    second: number;
    intensity: number;
}

I believe my approach might be incorrect, but I am struggling to identify the issue. Can someone point out what exactly is causing the problem?

Answer №1

The errors are essentially indicating two key points:

  • The first error implies that you're attempting to assign a value of type MqttPublishAnchorPoint to a field of type MqttPublishAnchorPoint[]. This suggests that TypeScript is interpreting your array as something other than an array, but rather as a singular MqttPublishAnchorPoint.
  • The second error highlights the fact that you're trying to convert a value of array type (undefined[]) into MqttPublishAnchorPoint, which causes confusion for TypeScript due to the conflicting types.

Both of these errors stem from the casting issue: <T>[] actually means [] as T, not [] as T[], meaning the cast is applied to the array itself and not its internal type. The solution is straightforward:

const publishObject: MqttPublishObject = {
  IsTurnedOff: detail.isTurnedOff,
  processType: detail.processType.name === "linear" ? 0 : 1,
  anchorPoints: <MqttPublishAnchorPoint[]>[],
};

Playground (simplified by removing unnecessary details).


However, in this scenario, the cast is not required at all. Since the variable is strongly typed, TypeScript can deduce the array type without needing it explicitly mentioned next to the value:

const publishObject: MqttPublishObject = {
  IsTurnedOff: detail.isTurnedOff,
  processType: detail.processType.name === "linear" ? 0 : 1,
  anchorPoints: [], // inferred as MqttPublishAnchorPoint[]
};

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