This question about JavaScript with function-Object.
I have a function like:
function person(){
this.name = 'Tom',
this.Age //not set any value
}
And I created a object by "person" function:
var obj = new person();
obj.name //Tom
obj.Age //undefined
obj.DOB //also returned undefined
How to distinguish "Age” property already exist in "obj" but no value, or "DOB" does not exist at all.
JavaScript distinguishes between null, which is a value that indicates a deliberate non-value (and is only accessible through the null keyword), and undefined, which is a value of type undefined that indicates an uninitialized value — that is, a value hasn't even been assigned yet.
You can simply use
if(this.hasOwnProperty('Age')){
}
Or
if('Age' in this){
}
Age is anywhere in the object including in the prototype. The first will only be true if Age is a direct property on the object (and not only on the prototype).undefined, so you can't test for undefined to see if the property exists or not. .hasOwnProperty() is probably the best choice to test if a given property has been specifically assigned to this object or not.
'Age' property, it does not function, so initialization is mandatory
obj.Age is not already existing. if you wanted it to then you have to initialize it to null or undefined
then you could check like
if(obj.hasOwnProperty('Age')){
}
You only want to check for it's existence? Use an if statement
if (obj.Age) {
// do stuf
} else {
// do other stuff
}
undefined returns as a falsy value to the if statement, so if it's not there it will execute the else statement. If it does exist, then the first block.
