how to update value of property of an object through argument variable

Question

I would like to increment property SCountDown of an object pomodoro with method customization, which has an argument chooseSession and this method is called within sessionPlus method passing SCountDown as an argument. when i call sessionPlus method , SCountDown property increments for once and when it is called for second time and so on , It doesn't increment.

Here's my code:

pomodoro = {   
 SCountDown: 0,
 customization: function(chooseSession) {
    chooseSession++;
     console.log(chooseSession);       
 },
 sessionPlus : function() {
    this.customization(this.SCountDown);        
 }
}
pomodoro.sessionPlus(); // it returns 1 , this is what i needed but
pomodoro.sessionPlus(); // it also returns 1, but what i need is 2.

You cannot pass variables by reference in JavaScript; [all variables are passed by value](http://stackoverflow.com/questions/6605640/javascript-by-reference-vs-by-value). If you want to pass a variable into a function and have that function increment it, you need to pass in an object and increment one of its properties, or rethink your solution so that a pass-by-reference is not required. — user229044, Dec 24 '15 at 04:22

score 2 · Answer 1 · answered Dec 24 '15 at 04:22

JavaScript uses pass by sharing and since SCountDown is a primitive value (integer) it cannot be referenced in the way you are attempting to. It's always going to log as 1 because you will always be passing in the primitive value 0 and then incrementing that. You need to reference this.SCountDown directly instead of chooseSession.

var pomodoro = {
    SCountDown: 0,
    customization: function(chooseSession) {
        console.log(chooseSession);
    },
    sessionPlus: function() {
        this.SCountDown++;
        this.customization(this.SCountDown);
    }
}

pomodoro.sessionPlus();
pomodoro.sessionPlus();

score 1 · Answer 2 · answered Dec 24 '15 at 04:20

Try this code. You need increment this.SCountDown not chooseSession

 pomodoro = {
   SCountDown: 0,
   customization: function(chooseSession) {
     console.log(chooseSession);
   },
   sessionPlus: function() {
     this.SCountDown++;
     this.customization(this.SCountDown);
   }
 }
 pomodoro.sessionPlus();
 pomodoro.sessionPlus();

Hope this will help you.

score 1 · Accepted Answer · answered Dec 24 '15 at 04:22

When you pass this.SCountDown to customization, you pass its value (which is zero). Then, this value is incremented by 1 and it is outputed. The actual value of this.SCountDown never changes.

JavaScript passes primitives by value.
It means that there is no way to modify the primitive by passing it to a function.

function modify(a)
{
    a = a * 2;
}

var x = 2;
modify(x);
console.log(x); // 2

However, you can modify the inner properties of passed objects since JS uses call-by-sharing strategy.
Therefore, you can do the following:

function modify(o)
{
    o.val = o.val * 2;
}

var x = { val: 2 };
modify(x);
console.log(x.val); // 4

score 1 · Answer 4 · answered Dec 24 '15 at 05:09

pomodoro = {   
 SCountDown: 0,
 customization: function() {
 
     console.log(this.SCountDown);       
 },
 sessionPlus : function() {
    this.SCountDown=this.SCountDown+1;
    this.customization();        
 }
}
pomodoro.sessionPlus(); // it returns 1 , this is what i needed but
pomodoro.sessionPlus(); // it also returns 1, but what i need is 2.

May this help

how to update value of property of an object through argument variable

4 Answers4