Typescript converting JSON to string - Type string is not assignable to type = '{ .. }'

Question

I have this code:

interface ISource extends IdModel {
  source_type_id: number;
  network_id: number;
  company_connection_id: number;
  feed_id: number;
  connection_id: number;
  feed_ids: number[];
  name: string;
  tag: string;
  additional_data: ISourceData;
  default_user_image?: File;
  hasNewImage?: boolean;
  next_update?: string;
  post_field_overrides?: {
    user_full_name?: string;
    user_screen_name?: string;
    url?: string;
    network_id?: number;
  };
}

save() {
   //converting this to string because I am saving this in my database.
   this.source.post_field_overrides = JSON.stringify(this.source.post_field_overrides);

}

If I don't convert that into string, that value that's being saved in my DB is [Object object].

If I also convert that into String, I am getting this error/warning:

TS2322: Type 'string' is not assignable to type '{ user_full_name?: string | undefined; user_screen_name?: string | undefined; url?: string | undefined; network_id?: number | undefined; } | undefined'.

Any help on this, please? Thanks in advance!

post_field_overrides? is neither string nor any... You should decide what you want it to be and write that I guess... — Alexei Levenkov
– Alexei Levenkov, Commented Dec 4, 2020 at 3:19
did you try this?: JSON.stringify(JSON.parse(this.source.post_field_overrides)); — sonEtLumiere
– sonEtLumiere, Commented Dec 4, 2020 at 3:23

Eason Yu · Accepted Answer · 2020-12-04 03:24:32Z

2

this.source.post_field_overrides = JSON.stringify(this.source.post_field_overrides);

you have defined post_field_overrides as an object containing

{
    user_full_name?: string;
    user_screen_name?: string;
    url?: string;
    network_id?: number;
  }

in your typescript interface. now you want to replace the same atrribute with a string. this won't pass typescript's static type check.

if you need it to be a string , add another attribute in your interface.

or as Alexei Levenkov said ,use this instead to support both object and string

post_field_overrides?: string | {
    user_full_name?: string;
    user_screen_name?: string;
    url?: string;
    network_id?: number;
  };

answered Dec 4, 2020 at 3:24

Eason Yu

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2 Comments

PinoyStackOverflower Over a year ago

This one is really good! Thanks for this! Another problem is when I retrieve the data from the database, since it is a string, i have to JSON.parse() it the moment I retrieved it. This is how I parse it - this.source.post_field_overrides = JSON.parse(this.source.post_field_overrides);, The problem with that is I am getting again the error TS2345: Argument of type 'string | - how to do that?

Eason Yu Over a year ago

I guess when you use JSON.parse to parse the string into object, typescript type checking will just see it as JSON Object not type { user_full_name: ...} you defined in the interface. So you may need to abstract this type definition as an independent type PostFieldOverride, use it in the interface as well as this place with JSON.parse(this.source.post_field_overrides) as PostFieldOverride

Sarabadu · Accepted Answer · 2020-12-04 03:25:07Z

0

You are declaring post_field_overrides as object

 post_field_overrides?: {
    user_full_name?: string;
    user_screen_name?: string;
    url?: string;
    network_id?: number;
  };

if you need to use it as an String you need to declare as:

 post_field_overrides?: string;

answered Dec 4, 2020 at 3:25

Sarabadu

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