Javascript – Why is document.write considered a “bad practice”

javascript

I know document.write is considered bad practice; and I'm hoping to compile a list of reasons to submit to a 3rd party vendor as to why they shouldn't use document.write in implementations of their analytics code.

Please include your reason for claiming document.write as a bad practice below.

Best Answer

A few of the more serious problems:

document.write (henceforth DW) does not work in XHTML
~~DW does not directly modify the DOM, preventing further manipulation~~ (trying to find evidence of this, but it's at best situational)
DW executed after the page has finished loading will overwrite the page, or write a new page, or not work
DW executes where encountered: it cannot inject at a given node point
DW is effectively writing serialised text which is not the way the DOM works conceptually, and is an easy way to create bugs (.innerHTML has the same problem)

Far better to use the safe and DOM friendly DOM manipulation methods

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Javascript – Why don’t self-closing script elements work

XHTML 1 specification says:

С.3. Element Minimization and Empty Element Content

Given an empty instance of an element whose content model is not EMPTY (for example, an empty title or paragraph) do not use the minimized form (e.g. use <p> </p> and not <p />).

XHTML DTD specifies script elements as:

<!-- script statements, which may include CDATA sections -->
<!ELEMENT script (#PCDATA)>

Javascript – Why is using the JavaScript eval function a bad idea

Improper use of eval opens up your code for injection attacks
Debugging can be more challenging (no line numbers, etc.)
eval'd code executes slower (no opportunity to compile/cache eval'd code)

Edit: As @Jeff Walden points out in comments, #3 is less true today than it was in 2008. However, while some caching of compiled scripts may happen this will only be limited to scripts that are eval'd repeated with no modification. A more likely scenario is that you are eval'ing scripts that have undergone slight modification each time and as such could not be cached. Let's just say that SOME eval'd code executes more slowly.

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Javascript – Why don’t self-closing script elements work

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