TypeScript type guards for an interface

002-interface.ts

// In this made up scenario we are trying to make sure we didn't get
// a corrupted piece of data from a WebSocket
interface WebSocketMessage {
  time: number;
  text: string;
  description?: string;
  content: string[];
}

// You could also write this as one looong if statement if you prefer
const isWebSocketMessage = (value: unknown): value is WebSocketMessage => {
  if (!value) return false;
  if (typeof value.time !== 'number') return false;
  if (typeof value.text !== 'string') return false;
  if (typeof value.description !== 'string' && value.description !== undefined) return false;
  if (!Array.isArray(value.content) || !value.content.every(content => typeof content === 'string')) return false;

  return true;
}