What is the use of declaring variable inside constructor

Question

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?

this creates "instance" data - - data that is part of the instance of the object you make. let, var, & const create regular variables that do not store their data along with the instance and therefore you can't access their information from the instance variable. They exist as ways to create "private" data. — Scott Marcus
– Scott Marcus, Commented Apr 3, 2019 at 17:17

Scott Marcus · Accepted Answer · 2019-04-03 17:21:17Z

3

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.

edited Apr 3, 2019 at 17:21

Scott Marcus

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answered Apr 3, 2019 at 17:18

Mudassir

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Comments

Amir · Accepted Answer · 2019-04-03 17:19:23Z

2

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();

answered Apr 3, 2019 at 17:19

Amir

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1 Comment

Amon Over a year ago

Yup for me I was setting the default value of a this.state.startDate to the current date

user3579536 · Accepted Answer · 2019-04-03 18:36:00Z

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()

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