卫语句 Guard Clause

卫语句 Guard Clause 是一种编程模式用于在函数或方法开始时检查前置条件，如果条件不满足就提前返回或抛出异常。

可以使代码更清晰、减少嵌套层次，并提高可读性。

传统嵌套示例：

function getEmployeeSalary(employee) {
	if (employee != null){
		if (employee.department != null) {
			if (employee.department.financeRecords != null) {
			return employee.department.financeRecords.salary;
			}
		}
	}
		return 0;
}

使用卫语句：

function getEmployeeSalary(employee) {
  if (employee == null) return 0;
  if (employee.department == null) return 0;
  if (employee.department.financeRecords == null) return 0;
  
  return employee.department.financeRecords.salary;
}