CODEWITHSUNDEEP
javascript
Alwaysblue
Alwaysblue

Reputation: 11830

What is the use of declaring variable inside constructor

I was studying class based structure in Javascript

To say the least, I created a simple class 

class something { 
  constructor() {
  var name = "somexyz"
  this.age = 13 
  } 

  somefunc () {
    console.log(this.name)//Undefined
    console.log(name) //blank space
    console.log(this.age) //13
  }
}

And then call it using 

let foo = new something()
foo.somefunc()

Which consoles.log (mentioned in comments of above code) 1. undefined 2. empty space 3. 13

or take a constructor function 

function something () {
  this.age =  11
  let age = 13 
  this.getAge = function () {
   return this.age
  }
}

Here, 

let somethingNew = new something()

Will return age as 11

Now, my question is what exactly is the purpose of having variable in constructors then?

Upvotes: 4

Views: 12457

Answers (3)

user3579536
user3579536

Reputation:

  • vars in constructor are only defined in the scope of the constructor
  • In this case "" is a string that is present in the global scope. Not finding name in the current scope of the method execution, it is finded moved up to the window.name. Try to change name with something else (ex: name2) and You'll get an error.
  • For logging "somexyz" see the snippet below
  • Of course, the local variables in the constructor should be useful for the logic of the constructor itself

class something {
  
  constructor() {
    var name = "somexyz"
    var name2 = "somexyz"
    this.age = 13
  }

  somefunc () {
    console.log(this.name)//undefined 
    console.log(name) //window.name
    console.log(window.name) //window.name
    console.log(this.__proto__.constructor.name) //"something"
    console.log(this.age) //13
  }
}
let foo = new something()
foo.somefunc()

Upvotes: 1

Mudassir
Mudassir

Reputation: 1156

The emphasis should be on "simple class" - the notion of a local variable seems pointless but it's actually very useful.

Simple example - when you reuse a variable repeatedly

class transport
{
    constructor ()
    {
      const someString = "IUseThisAgainAndAgain";
      this.type = 'someType';
      this.name = 'someName';
      this.serial = someString + "SOMETHING";
      this.model = someString + "something thing";
      this.example = someString + "another string concat";
    }
}

As you can see we use someString alot. And you will see constructor method that are huge and having a variable for a repeated instance will allow better readability of code and allow you to easily modify the instance in one place.

Upvotes: 3

Amir
Amir

Reputation: 721

You can use variables to set a default value also for checking purposes or mathematical calculations.

For example:

class something { 
    constructor(age) {
      var default_age = "13";
      if(age > 13){
       this.age = age;
      }else{
       this.age = default_age;
      }
    } 

    myage() {
       console.log("I am "+this.age+" years old.")
    }
}
    
 var newobj = new something(15);
 newobj.myage();

 var newobj2 = new something(8);
 newobj2.myage();

Upvotes: 2

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