Utilize an array of JSON objects to populate an array of interfaces in Angular/Typescript

I am currently facing a dilemma - my code is functioning without any errors when executed, but my text editor is flagging an issue stating that the property 'categories' does not exist on type 'CategoryInterface[]' (specifically on the line where response.categories is assigned to a variable). This has left me questioning whether I am implementing things correctly.

    public categories: CategoryInterface[];

          .subscribe((response: CategoryInterface[]) => {
          this.categories = response.categories;
          console.log(this.categories);
      });

The data returned from my backend looks like this:

{
"categories": [
    {
        "categoryId": 1,
        "name": "Important",
        "description": "This category is important.",
        "order": 1,
        "createdBy": null,
        "createdAt": "2017-11-25 12:09:04",
        "updatedBy": null,
        "updatedAt": "2018-01-17 23:53:25",
        "categoryBoards": [
            {
                "categoryBoardId": 1,
                "categoryId": 1,
                "name": "Announcements",
                "description": null,
                "order": 2,
                "createdBy": null,
                "createdAt": "2017-11-25 12:09:49",
                "updatedBy": null,
                "updatedAt": "2018-01-18 00:09:02"
            },
            {
                "categoryBoardId": 23,
                "categoryId": 1,
                "name": "Rules",
                "description": null,
                "order": 1,
                "createdBy": null,
                "createdAt": "2018-01-18 00:08:57",
                "updatedBy": null,
                "updatedAt": "2018-01-19 00:05:51"
            }
        ]
    }
]

}

Answer №1

When converting your api response to an array of CategoryInterface, remember to utilize the subscribe method as shown below:

.subscribe((response: any) => {
    this.categories = <CategoryInterface[]> response.categories;
    console.log(this.categories);
});

Make sure to cast your api response categories to CategoryInterface[].

Extra Tip: According to the Angular style-guide, it is recommended to use classes instead of interfaces when declaring, and you can drop 'Interface' from the class name like changing CategoryInterface to just Category.

Answer №2

The reason for the error is that you have defined response as an array of type CategoryInterface[], while trying to access response.categories, which is actually the array of CategoryInterface[]. In reality, response just acts as a container for the array. When TypeScript is transpiled to JavaScript, all type information is removed, allowing it to work correctly during runtime.

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