Question

Is it possible in javascript to set properties inside methods?

For example

function Main() {

   this.method = function() {
      this.parameter = 'something_relevant'
   }
}

var p = new Main()
p.method()
console.log(p.method.parameter)

I tried this and it logs ‘undefined’. Is it about scope?

Answer 1

Inside method() you’re setting a property of the object on which the method is called rather than on the function object representing the method.

This shows the difference inside the method:

this.method = function() {
   this.parameter = 'abc'; // Set parameter on the object on which method() is called
   this.method.parameter = 'xyz'; // Set parameter on the object representing the method itself
};

This shows the difference in accessing the property after the method is called

p.method();
console.log(p.parameter); // Display property of the object p, equals 'abc'
console.log(p.method.parameter); // Display property of the function object representing method(), equals 'xyz'

You should decide whether you need a property on the function object or on p object. Note that the function object may be shared by a number of objects created by the Main() constructor. Hence it will behave in a manner somewhat similar to a static member in languages like C++ or Java.

If you intend to use property defined on the object, your code should look similar to this:

function Main() {

   this.method = function() {
      this.parameter = 'something_relevant'; // Set property on object on which method() is called.
   };
}

var p = new Main();
p.method();
console.log(p.parameter); // Read property from object p.

If you intend to use a property defined on the function object representing method(), your code should look similar to this:

function Main() {

   this.method = function() {
      this.method.parameter = 'something_relevant'; // Set property on function object representing method().
   };
}

var p = new Main();
p.method();
console.log(p.method.parameter); // Read property from the function object.