Exploring the concept of scoping in two brief JavaScript functions

Question

Exploring the concept of scoping in two brief JavaScript functions

Can you clarify the distinction between the following pair of JavaScript functions? I understand that variables declared with var are local within the function, and those declared with 'this' keyword are accessible in the outer scope. Are there any other disparities between

function student(param1, param2, param3) {
    this.name = param1;
    this.age = param2;
    this.address = param3;
}

and

function student(param1, param2, param3) {
    var name = param1;
    var age = param2;
    var address = param3;
}

javascript performance javascript-objects

Answer 1

Answer №1

Answer: The first function is used as a constructor to create objects. The second function does not serve any purpose. If you want to implement 'private variables', you can use functional scoped variables in the constructor and access them through instance methods using closures.

To create a student object, you would use the first function by passing parameters that will be assigned to the newly created student.

function student(param1, param2, param3){
this.name = param1;
this.age = param2;
this.address = param3;
}

var student1 = new student('steve', 22,'333 E 3rd Ave');
var student2 = new student('rachel', 34,'111 N 1st St');

console.log(student1.name); // 'steve'
console.log(student2.name); // 'rachel'

The second function does not fulfill the role of a constructor. It declares variables with functional scope which are not utilized, and they will be garbage collected once the function execution ends.

If you attempt to use the second function like a constructor, it will not work:

function student(param1, param2, param3){
var name = param1;
var age = param2;
var address = param3;
}

var badStudent = new student('Greg', 22,'222 W 2nd Rd');

console.log(badStudent.name); // undefined

Edit

A method to achieve 'private member variables' using variables declared with var inside a constructor is to use closures:

function student(param1, param2, param3) {
var name = param1;
var age = param2;
var address = param3;

this.getName = function(newName) {

if (newName)
name = newName;

return name;
};
}

var myStudent = new student('steve', 22,'333 E 3rd Ave');

console.log(myStudent.name);

console.log(myStudent.getName());

console.log(myStudent.getName('dan'));

console.log(myStudent.getName());

By using closures, the instance method getName retains references to the variables declared in the constructor even after its execution. This allows you to access and modify these 'private' variables through the instance method.

Answer 2

Answer: The first function is used as a constructor to create objects. The second function does not serve any purpose. If you want to implement 'private variables', you can use functional scoped variables in the constructor and access them through instance methods using closures.

To create a student object, you would use the first function by passing parameters that will be assigned to the newly created student.

function student(param1, param2, param3){
this.name = param1;
this.age = param2;
this.address = param3;
}

var student1 = new student('steve', 22,'333 E 3rd Ave');
var student2 = new student('rachel', 34,'111 N 1st St');

console.log(student1.name); // 'steve'
console.log(student2.name); // 'rachel'

The second function does not fulfill the role of a constructor. It declares variables with functional scope which are not utilized, and they will be garbage collected once the function execution ends.

If you attempt to use the second function like a constructor, it will not work:

function student(param1, param2, param3){
var name = param1;
var age = param2;
var address = param3;
}

var badStudent = new student('Greg', 22,'222 W 2nd Rd');

console.log(badStudent.name); // undefined

Edit

A method to achieve 'private member variables' using variables declared with var inside a constructor is to use closures:

function student(param1, param2, param3) {
var name = param1;
var age = param2;
var address = param3;

this.getName = function(newName) {

if (newName)
name = newName;

return name;
};
}

var myStudent = new student('steve', 22,'333 E 3rd Ave');

console.log(myStudent.name);

console.log(myStudent.getName());

console.log(myStudent.getName('dan'));

console.log(myStudent.getName());

By using closures, the instance method getName retains references to the variables declared in the constructor even after its execution. This allows you to access and modify these 'private' variables through the instance method.

Answer 3

Answer №2

The main distinction between variables declared with this and var lies in their scope. Variables assigned to this will become part of objects, while those declared with var may be considered private. This is not always guaranteed, as it depends on the use of return. If return is not utilized, then they will indeed be private.

Example

function Person(firstName, lastName, dob) {
  var _firstName = firstName, 
      _lastName = lastName,
      _dob = new Date(dob);
  
  this.fullName = _firstName + " " + _lastName;
  this.age = (new Date()).getFullYear() - _dob.getFullYear();
}

var person = new Person('John', 'Doe', '1980/05/25');
console.log(person);
console.log(person._firstName)

In the above code snippet, the variable _firstName was created using var, making its scope limited to the function only. Consequently, attempting to access it outside the function will result in an error.

To summarize, this can be used to define public properties, while var can be used for private ones.

Answer 4

The main distinction between variables declared with this and var lies in their scope. Variables assigned to this will become part of objects, while those declared with var may be considered private. This is not always guaranteed, as it depends on the use of return. If return is not utilized, then they will indeed be private.

Example

function Person(firstName, lastName, dob) {
  var _firstName = firstName, 
      _lastName = lastName,
      _dob = new Date(dob);
  
  this.fullName = _firstName + " " + _lastName;
  this.age = (new Date()).getFullYear() - _dob.getFullYear();
}

var person = new Person('John', 'Doe', '1980/05/25');
console.log(person);
console.log(person._firstName)

In the above code snippet, the variable _firstName was created using var, making its scope limited to the function only. Consequently, attempting to access it outside the function will result in an error.

To summarize, this can be used to define public properties, while var can be used for private ones.

Answer 5

Answer №3

When it comes to creating objects in JavaScript, we typically use the 'function' keyword as a constructor. These constructor functions are responsible for returning an object. If you declare a variable using 'var' instead of 'this.varName', it means you are attempting to use those variables to create an object. Variables declared with 'var' are simply local variables within the function scope.

On the other hand, when you use 'this.variableName', you are actually creating a property for the object that the constructor function is trying to instantiate.

'this.' refers to the object being created by the constructor function. 'var variableName' declares a local variable that is not a property of the 'this' object.

function student(param1, param2, param3) {
    this.name = param1;
    this.age = param2;
    this.address = param3;
}

var t = new student('farhad', 28, 'address');

This will create the following object:

t{
   name: 'farhad',
   age: 28,
   address: 'address'
}

However, if we look at the following example:

function student2(param1, param2, param3) {
    var name = param1;
    var age = param2;
    var address = param3;
}
var t2 = new student2('farhad', 28, 'address');

We'll see that the object 't2' does not contain any properties.

Answer 6

When it comes to creating objects in JavaScript, we typically use the 'function' keyword as a constructor. These constructor functions are responsible for returning an object. If you declare a variable using 'var' instead of 'this.varName', it means you are attempting to use those variables to create an object. Variables declared with 'var' are simply local variables within the function scope.

On the other hand, when you use 'this.variableName', you are actually creating a property for the object that the constructor function is trying to instantiate.

'this.' refers to the object being created by the constructor function. 'var variableName' declares a local variable that is not a property of the 'this' object.

function student(param1, param2, param3) {
    this.name = param1;
    this.age = param2;
    this.address = param3;
}

var t = new student('farhad', 28, 'address');

This will create the following object:

t{
   name: 'farhad',
   age: 28,
   address: 'address'
}

However, if we look at the following example:

function student2(param1, param2, param3) {
    var name = param1;
    var age = param2;
    var address = param3;
}
var t2 = new student2('farhad', 28, 'address');

We'll see that the object 't2' does not contain any properties.

Answer 7

Answer №4

this is a crucial keyword utilized within functions that holds the value of the object triggering the function.

When used, this specifically points to the particular instance of the object calling the function and doesn't receive a value until the respective object initiates the function in which it's declared.

function Employee(name, age, address) {
      this.firstName = name;
      this.employeeAge = age;
      this.homeAddress = address;
      this.introduce = function() {
        console.log(this.name + this.age + this.address)
      }
    }
    var employee1 = new Employee('John', 30, '123 Main St');
    employee1.introduce(); // John30Main St

In contrast, with

var firstName=name; var employeeAge=age; var homeAddress=address;

, each parameter is being assigned to a variable, with these variables limited in scope solely inside the function itself.

Answer 8

this is a crucial keyword utilized within functions that holds the value of the object triggering the function.

When used, this specifically points to the particular instance of the object calling the function and doesn't receive a value until the respective object initiates the function in which it's declared.

function Employee(name, age, address) {
      this.firstName = name;
      this.employeeAge = age;
      this.homeAddress = address;
      this.introduce = function() {
        console.log(this.name + this.age + this.address)
      }
    }
    var employee1 = new Employee('John', 30, '123 Main St');
    employee1.introduce(); // John30Main St

In contrast, with

var firstName=name; var employeeAge=age; var homeAddress=address;

, each parameter is being assigned to a variable, with these variables limited in scope solely inside the function itself.

Exploring the concept of scoping in two brief JavaScript functions

Answer №1

Answer №2

Example

Answer №3

Answer №4

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