Is there an option to not create an object with particular condition within constructor, e.g.
function Monster(name, hp) {
if (hp < 1) {
delete this;
}
else {
this.name = name;
}
}
var theMonster = new Monster("Sulley", -5); // undefined
I think what you're supposed to do is throw an exception.
function Monster(name, hp) {
if (hp < 1) {
throw "health points cannot be less than 1";
}
this.hp = hp;
this.name = name;
}
var m = new Monster("Not a good monster", 0);
A function called as a constructor (with the new operator) will always return an instance unless it explicitly returns an object. You can therefore return an empty object, and use the instanceof operator to check what came back:
function Monster(name, hp) {
if (hp < 1) {
return {};
}
else {
this.name = name;
}
}
var theMonster = new Monster("Sulley", -5);
console.log(theMonster instanceof Monster); // false
This behaviour is explained in the spec (13.2.2):
8. Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing the argument list passed into [[Construct]] as args.
9. If Type(result) is Object then return result.
10. Return obj.
However, as others have pointed out, whether you should actually do this is questionable.
It does not make sense, you are trying to stop construction of an object during its construction phase. The better way would be to use something suggested by @Amberlamps or use something like factory pattern to create objects.
Monsterfunction? You cannot usetheMonsteranyway if you don´t want it to be an object. You could addthis.isMonster = (hp >= 1);to yourMonsterfunction.