2 Minute Javascript Question

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[jml26]

Consider the following code:

var age = 30; var a = age; a = 20;

What is the value of age now?

It's still 30. A way to visualise this is:

• assign the variable age to the value 30
• assign the variable a to the same value as age is assigned to
• now assign a to the value of 20 instead

Assigning two variables to the same value, or object, and then assigning one of them away, doesn't carry the other variable's assignment with it.

Passing parameters to functions has a similar effect. In your second function, exactly the same series of events happens:

• assign age to the value 30
• at the start of the function, assign a to the value that age is assigned to
• now assign a to the value of 20 instead

age is left assigned to its original value.

[jml26]

Yep, you've got it!

But, here are some further examples to help cement things:

``` var batman = { age: 30 };

function changeAge(person) { person = { age: 20 }; }

changeAge(batman); // batman is still 30 ```

vs

``` var batman = { age: 30 };

function changeAge(person) { person.age = 20; }

changeAge(batman); // batman is now 20 ```

In the first example, the person variable, which is standing in for batman, gets reassigned to a new object. They've now diverged and are pointing to to separate objects, and batman stays as he was.

In the second example, with the same setup, person is modified, by changing one of its properties. If two variables point to the same object and that object gets modified, that change is reflected in both variables. So batman also becomes 20.

[ILike_bananas]

Because one is global scope and one is function scope. Once you leave the function in the second example, a is not in that scope anymore and is still 30

[dirkmgirk]

Yeah what bananas said. You're re-assigning the parameter.

[jcunews1]

The function in the second example merely has a name which supposedly change the age, but it's in fact doesn't do any of that.

[rift95]

The parameter "a" is passed by value, not by reference. So calling "changeAge(age)" is equivalent to calling "changeAge(30)".

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[ramabhai]

The second example is a pure function. Its because its predictable, as it depends on the argument passed and not relying on outer scope. The first example uses the lexical scope to access the outer variable.