Validate before data access

Always validate data existence and type before accessing properties or methods to prevent runtime errors from null/undefined values. This includes:

1. Array bounds checking before indexing
2. Object property existence verification
3. Type validation before method calls
4. Use of optional chaining and nullish coalescing operators

Example of unsafe code:

function toolCallStateToSystemToolCall(state: ToolCallState): string {
  return state.toolCall.function.name;  // Unsafe - multiple potential null points
}

Safe version:

function toolCallStateToSystemToolCall(state: ToolCallState): string {
  if (!state?.toolCall?.function) {
    throw new Error("Invalid tool call state");
  }
  return state.toolCall.function.name ?? "unknown";
}

Key practices:

• Use optional chaining (?.) for nested object access
• Provide default values using nullish coalescing (??)
• Add explicit null checks for critical operations
• Validate array indices before access
• Check object type and property existence before destructuring

This prevents the most common causes of runtime errors and improves code reliability.