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() {
...