check null,empty or undefined angularjs

just use -

if(!a) // if a is negative,undefined,null,empty value then...
{
    // do whatever
}
else {
    // do whatever
}

this works because of the == difference from === in javascript, which converts some values to "equal" values in other types to check for equality, as opposed for === which simply checks if the values equal. so basically the == operator know to convert the "", null, undefined to a false value. which is exactly what you need.

You can do

if($scope.test == null || $scope.test === ""){
  // null == undefined
}

if false, 0 and NaN can also be considered as false values you can just do

if($scope.test){
 //not any of the above
}