import { JSONAPIResourceObject } from './jsonapi-resource-object.model';

export class JSONAPIResponse<T extends JSONAPIResourceObject | JSONAPIResourceObject[]> {
    public data: T;
    public included: JSONAPIResourceObject[];

    public constructor(json: {data: any, included?: any}) {
        this.data = json.data;
        this.included = json.included;
    }

    public toPayload(): T {
        return this.data;
    }

    public toIncluded(): JSONAPIResourceObject | JSONAPIResourceObject[] {
        return this.included;
    }

    public toIncludedByType<U extends JSONAPIResourceObject>(type: string): U[] {
      if (!this.included) {
        return [];
      }
      return <U[]>this.included.filter((include: JSONAPIResourceObject) => include.type === type);
    }

    public toJSON(): {data: T, included?: JSONAPIResourceObject | JSONAPIResourceObject[]} {
        if (this.included) {
            return {data: this.data, included: this.included};
        } else {
            return {data: this.data};
        }
    }
}

import { JSONAPIResourceObject } from './jsonapi-resource-object.model';

export class JSONAPIResponse<T extends JSONAPIResourceObject | JSONAPIResourceObject[]> {
    public data: T;
    public included: JSONAPIResourceObject[];

    public constructor(json: {data: any, included?: any}) {
        this.data = json.data;
        this.included = json.included;
    }

    public toPayload(): T {
        return this.data;
    }

    public toIncluded(): JSONAPIResourceObject | JSONAPIResourceObject[] {
        return this.included;
    }

    public toIncludedByType<U extends JSONAPIResourceObject>(type: string): U[] {
        return <U[]>this.included.filter((include: JSONAPIResourceObject) => include.type === type);
    }

    public toJSON(): {data: T, included?: JSONAPIResourceObject | JSONAPIResourceObject[]} {
        if (this.included) {
            return {data: this.data, included: this.included};
        } else {
            return {data: this.data};
        }
    }
}
--- 
+++ 
@@ -18,7 +18,10 @@
     }
 
     public toIncludedByType<U extends JSONAPIResourceObject>(type: string): U[] {
-        return <U[]>this.included.filter((include: JSONAPIResourceObject) => include.type === type);
+      if (!this.included) {
+        return [];
+      }
+      return <U[]>this.included.filter((include: JSONAPIResourceObject) => include.type === type);
     }
 
     public toJSON(): {data: T, included?: JSONAPIResourceObject | JSONAPIResourceObject[]} {
Return empty array when include not defined
