Single responsibility principle

The single responsibility principle declares that a class should have one, and only one reason to change. And the definition of the world reason in this sentence is important.

Example

Consider a Command class that is designed to work with both command-line interface and graphical user interface:

class Command { 
  environment: Environment; 
 
  print(items: ListItem[]) { 
    let stdout = this.environment.stdout; 
   
    stdout.write('Items:\n'); 
   
    for (let item of items) { 
      stdout.write(item.text + '\n'); 
    } 
  } 
   
  render(items: ListItem[]) { 
    let element = <List items={items}></List>; 
    this.environment.render(element); 
  } 
   
  execute() { } 
} 

To make this actually work, execute method would need to handle both the command execution and result displaying:

class Command { 
  .. 
   
  execute() {
 ...