Proper way to declare JSON object in Typescript

Update: 7/26/2021

I revisited the discussion noted in the original answer, and there is now an update with an improved implementation.

type JSONValue = 
 | string
 | number
 | boolean
 | null
 | JSONValue[]
 | {[key: string]: JSONValue}

interface JSONObject {
  [k: string]: JSONValue
}
interface JSONArray extends Array<JSONValue> {}

This has been working very well for me.

Discussion reference: https://github.com/microsoft/TypeScript/issues/1897#issuecomment-822032151

Original answer: Sep 29 '20

I realize this is an old question, but I just found a solution that worked very well for me. Declare the following

type JsonPrimitive = string | number | boolean | null
interface JsonMap extends Record<string, JsonPrimitive | JsonArray | JsonMap> {}
interface JsonArray extends Array<JsonPrimitive | JsonArray | JsonMap> {}
type Json = JsonPrimitive | JsonMap | JsonArray

then any of the following (including the OP's version slightly modified for syntax errors) will work

    let a: Json = {};
    
    a[1] = 5;
    a["abc"] = "abc";
    a = {
      a: {
        a: 2,
      },
      b: [1, 2, 3],
      c: true,
    };
    a = [
      {
        "id": 1,
        "title": "something",
        "node": [
          {
            "id": 1,
            "title": "something",
            "node": [],
          },
        ],
      },
      {
        "id": 2,
        "title": "something",
        "node": [
          {
            "id": 1,
            "title": "something",
            "node": [],
          },
        ],
      },
    ];

This answer should be credited to Andrew Kaiser who made the suggestion on the discussion about making Json a basic type in Typescript: https://github.com/microsoft/TypeScript/issues/1897#issuecomment-648484759

Here is an easy and naive implementation of what you're asking for:

interface IDataNode {
    id: number;
    title: string;
    node: Array<IDataNode>;
}

If you want to instantiate said nodes from code:

class DataNode implements IDataNode {
    id: number;
    title: string;
    node: Array<IDataNode>;

    constructor(id: number, title: string, node?: Array<IDataNode>) {
        this.id = id;
        this.title = title;
        this.node = node || [];
    }

    addNode(node: IDataNode): void {
        this.node.push(node);
    }
}

Using this to hardcode your structure:

let data: Array<IDataNode> = [ 
    new DataNode(1, 'something', [
        new DataNode(2, 'something inner'),
        new DataNode(3, 'something more')
    ]),
    new DataNode(4, 'sibling 1'),
    new DataNode(5, 'sibling 2', [
        new DataNode(6, 'child'),
        new DataNode(7, 'another child', [
            new DataNode(8, 'even deeper nested')
        ])
    ])
];