Why are Javascript bookmarklets wrapped in closures?

Question

Why are Javascript bookmarklets wrapped in closures?
I wouldn't think someone would put an unnamed function in there for no reason.

I have read quite a few explanations on closures, but I still don't feel that i've grasped the whole concept.

Thanks guys!

Also, anonymous function use eliminates the need to suppress last return value with `void` operator. — OnTheFly, Jan 16 '12 at 05:21
@user539484 could you elaborate on your last comment, with an example? Thanks! — Web_Designer, Jan 16 '12 at 05:36
Or better see this doc: https://developer.mozilla.org/en/JavaScript/Reference/Operators/void#JavaScript_URIs — OnTheFly, Jan 16 '12 at 05:49
@user539484 Oh thanks, didn't know that. So this method: `javascript:(function(){var x=10; })()` ...would work, but this: `javascript: var x=10;` ...would attempt to go to that URL! — Web_Designer, Jan 16 '12 at 06:35
yep, if return value isnt undefined, bookmarklet will destroy a page — OnTheFly, Jan 16 '12 at 07:23
@OnTheFly Does that mean that using `javascript:void(my code)` would work as well? Does that leak variables? (If it works, I would love to see that as an answer to this question.) — Hjulle, Aug 31 '14 at 20:54

score 2 · Accepted Answer · answered Jan 16 '12 at 03:10

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To not contaminate the global scope by executing a function after creation.

(function(){var x=10; })()
alert(window.x) // undefined

answered Jan 16 '12 at 03:10

Ivan Castellanos

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score 1 · Answer 2 · answered Jan 16 '12 at 03:10

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This prevents variables in the bookmarklet from leaking into the page.

answered Jan 16 '12 at 03:10

SLaks

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score 1 · Answer 3 · answered Jan 16 '12 at 04:06

Two reasons:

contamination

The first is for preventing global contamination for any of the variables needed for the bookmarklet. Using a closure means that var a wont add window.a to the global context. Additionally, using a named function would add it to the global namespace. function a() {...} would add window.a.

accidental DOM recreation

The second is to avoid accidentally recreating the DOM by returning a string. Any javascript: url where a string is returned will create a brand new DOM using the contents of the string as the source.

score 0 · Answer 4 · answered Jan 16 '12 at 03:15

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Because the folks who are making them are good citizens of the web :) and don't want to step over the actual page's JavaScript accidentally.

answered Jan 16 '12 at 03:15

Rajat

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score 0 · Answer 5 · edited May 23 '17 at 11:55

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A mixture of scope conflicts and return policy.

zzzzBov's answer in this thread covers the main issues succinctly.

edited May 23 '17 at 11:55

Community

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answered Jan 16 '12 at 03:17

CBusBus

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score 0 · Answer 6 · answered Jan 16 '12 at 03:20

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It's generally to avoid polluting the global namespace, it's also known as the Module Pattern as described by Ben Cherry: http://www.adequatelygood.com/2010/3/JavaScript-Module-Pattern-In-Depth

answered Jan 16 '12 at 03:20

Ken Struys

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Why are Javascript bookmarklets wrapped in closures?

6 Answers6

contamination

accidental DOM recreation