Proper way to declare JSON object in Typescript

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')
        ])
    ])
];