Array getting reset in function

Question

Scenario 1

var a = [1, 2, 3];

function isChanged(a) {
  a = [];
}
isChanged(a);
console.log(a);

Scenario 2

var a = [1, 2, 3];

function isChanged(stack) {
  while (stack.length != 0) {
    stack.pop();
  }
}
isChanged(a);
console.log(a);

Why is the array not empty in the first function, but it is empty when I use the second one?

Edit :

I played around the changing the variable by assignment, and overriding its property.

Scenario 3 - changing the property of object

var a = {
  prop: "Stackoverflow"
}

function change(a) {
  a.prop = "stack"
}
change(a)
console.log(a)

Scenario 4 - changing the whole variable itself

var a = {
  prop: "Stackoverflow"
}

function change(a) {
  a = {
    "prop": "stack"
  }
}
change(a);
console.log(a);

@SpencerWieczorek. Is it because of this? Changing the value of a variable never changes the underlying primitive or object. However, changing a property of an object referenced by a variable does change the underlying object. - from the link you referenced in comment — netuser
– netuser, Commented Jun 18, 2017 at 3:23

Michael Geary · Accepted Answer · 2017-06-18 03:24:47Z

1

In the first example, when you set a = []; you are only changing the local variable a, which (after the assignment) no longer has any relation to the global variable that happens to have the same name. In the second example, you are directly modifying the stack variable, which happens to be the same object as the global variable a. If you were to do stack = whatever, it would behave similarly to the first example.

answered Jun 18, 2017 at 3:24

Michael Geary

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5 Comments

netuser Over a year ago

Thanks for answer. I didnt understand If you were to do stack = whatever, it would behave similarly to the first example. part. If I set stack = [], it would be similar to first example? - Sorry, my bad.

Michael Geary Over a year ago

Because that would change stack to be a reference to a new, different object. It would no longer refer to the same object as before.

Michael Geary Over a year ago

Another way to look at it: whenever you use =, you are changing the variable on the left to refer to a completely different thing than before. That's different from calling a method like pop(), which modifies the original object.

netuser Over a year ago

I played around it as you explained. So if I change the property, the it affects the parent scope, but if I use assignment, it creates a new variable? Is my example correct in question?

Michael Geary Over a year ago

Yes, it sounds like you have it right! Glad this was helpful.

Eric Lovell · Accepted Answer · 2017-06-18 03:16:02Z

0

My guess is because in the first a is an object reference, which is set to a new empty array. Where as the 2nd calls a function on that object reference, which will actually remove elements from the array you pass to your function.

answered Jun 18, 2017 at 3:16

Eric Lovell

1

1 Comment

netuser Over a year ago

But as far as I know, all the operations are contained in the function scope, it should not affect the parent scope. Isn't it?

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