SOURCE

const data = [
    {
        "id": "1",
        "sub": [
            {
            "id": "2",
            "sub": [
                {
                "id": "3",
                "sub": null
                },
                {
                "id": "4",
                "sub": [
                    {
                    "id": "6",
                    "sub": null
                    }
                ]
                },
                {
                "id": "5",
                "sub": null
                }
            ]
            }
        ]
    },
    {
        "id": "7",
        "sub": [
            {
                "id": "8",
                "sub": [
                    {
                        "id": "9",
                        "sub": [
                            {
                                id: "15",
                                sub: null
                            }
                        ]
                    }
                ]
            }
        ]
    },
    {
        "id": "10",
        "sub": null
    }
];

function findPath(data, id) {
    const result = [];

    function find(data, id) {
        const index = data.findIndex(item => item.id === id);
        if (index >= 0) {
            result.push(id);
            return result;
        }
        for (let i = 0; i < data.length; i++) {
            if (data[i].sub) {
                result.push(data[i].id);
                find(data[i].sub, id);
                if (result.includes(id)) {
                    return;
                }
                result.pop();
            }
        }
    }

    find(data, id);

    return result;
}

console.log(findPath(data, "2"));