For Javascript it looks like you're calling a process.stdout.write

However, it looks like this is an old syntax used by Nodejs or low level, You could replace this line with a simple console.log

class Graph {
    constructor(vertices) {
        this.V = vertices; // Number of vertices
        this.adjList = new Array(vertices).fill(null).map(() => []);
    }

    addEdge(u, v) {
        this.adjList[u].push(v);
        this.adjList[v].push(u); // For undirected graph
    }

    BFS(startVertex) {
        const visited = new Array(this.V).fill(false); // To keep track of visited vertices
        const queue = [];

        visited[startVertex] = true;
        queue.push(startVertex);

        while (queue.length !== 0) {
            const currentVertex = queue.shift();
            process.stdout.write(currentVertex + " ");

            // Explore adjacent vertices
            for (const neighbor of this.adjList[currentVertex]) {
                if (!visited[neighbor]) {
                    visited[neighbor] = true;
                    queue.push(neighbor);
                }
            }
        }
    }
}