Why variables are not automatically updated in Java Script? (scope? )

Question

I have a following code:

var obj = {
    car: "volvo",
    speed: 20
};

var x = obj.speed;
obj.speed = 500;
console.log(x);
console.log(obj.speed);

obj.speed = 1000;
var y = obj.speed;
console.log(y);

One would expect that x would be 500 at the time you log it to console, however it is still 20. Console.log(obj.speed) results in 500.

Can you let me know why it is like that?

I understand that if I swap theier places: var x declaration and obj.speed = 500, it will point to 500. Just like with y variable

But why? Does the code not check last value of x variable before logging it?

*"One would expect that x would be 500"* – No, one would not. You're assigning the value `20` to a variable, and then you're updating a different variable… — deceze, Mar 26 '18 at 11:07
See e.g. **https://codeburst.io/explaining-value-vs-reference-in-javascript-647a975e12a0** — Peter B, Mar 26 '18 at 11:10
This was my very first WTF moment, when I started programming. After an explanation like the above, it made sense. — Sergio Tulentsev, Mar 26 '18 at 11:18
Put the other way around: when exactly would you expect variables to become "detached"? It's not really workable if every variable keeps implicit connections to other variables. — deceze, Mar 26 '18 at 11:26

score 1 · Accepted Answer · answered Mar 26 '18 at 11:15

When you are assigning xto obj.speed, It is just similar to assigning a value to a primitive variable.

If you assign value of obj to variable y, then it will get updated as it will be referring to the same memory location.

var obj = {
    car: "volvo",
    speed: 20
};

// Object is a referece data type.
// But you are assigining value of obj.speed which is primitive to the the x
// Hence value of x won't change even if you change the value of obj.speed.

var x = obj.speed;
obj.speed = 500;
console.log(x);


// But if you assign the value of object to another variable `y`
// Now, y will be pointing to the same memory location as that of obj
// If you update the obj, then the value of y will also get updated
var y = obj;
obj.speed = 1000;
console.log(y);
console.log(obj);

Why variables are not automatically updated in Java Script? (scope? )

1 Answers1