Reaching a variable from nested function in Javascript

Question

Can somebody please explain me why the following code works?

function getLastName()
{
    fullName.lastName = "World";   
}

function writeName()
{
    fullName = {};
    fullName.firstName = "Hello";
    getLastName();
    document.write(fullName.firstName + " " + fullName.lastName);
}

writeName();

For some reason, getLastName() can reach local its enclosing method's local state. How can this work? And also should I utilize this feature of Javascript or it is considered as a bad practice? If it is a bad practice, could you please explain why?

You can see the actual code working here at http://jsbin.com/atituk/2/edit

I'd recommend to avoid this practice, as also most scenarios of global variables. Instead, define local vars, pass them to submethods, ... — evgenyl
– evgenyl, Commented Mar 29, 2013 at 18:18

adeneo · Accepted Answer · 2013-03-29 18:14:02Z

8

You don't have any local variables, that would require using the var keyword. All your variables are global and can be accessed anywhere within window, which is not considered good practice at all.

answered Mar 29, 2013 at 18:14

adeneo

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Quentin · Accepted Answer · 2013-03-29 18:14:22Z

2

You have not used the var keyword against fullName inside the writeName function, you are therefore taking it from the scope outside writeName. It continues up the chain until it reaches the outer most scope, at which point it creates a global.

Globals are, in general, bad practise as they are hard to keep track of and more likely to be overwritten by accident (e.g. in a race condition).

If you were using strict mode this would create an error instead of a global.

answered Mar 29, 2013 at 18:14

Quentin

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2 Comments

Tarik Over a year ago

I liked the fact that you mentioned "use strict".

Nelson Benítez León Over a year ago

+1 for mentioning how to avoid this situation by using strict.

Community · Accepted Answer · 2017-05-23 10:32:04Z

1

You are using all variables of yours as global variables. So all are recognized everywhere. In order to have a better understanding of variable scopes in Javascript have a look at this great example https://stackoverflow.com/a/500459/655316

edited May 23, 2017 at 10:32

CommunityBot

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answered Mar 29, 2013 at 18:21

Mostafa Shahverdy

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