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Why am i receiving this jsfiddle error, document.write can be a form of eval
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I am testing a code I found while reading a book. I get this error while testing it out in JS fiddle, document.write can be a form of eval.

     var text = '<html><body bgcolor=linen><p>' +
    'This is <b>bold<\/b>!<\/p><\/body><\/html>';

var tags = /[^<>]+|<(\/?)([A-Za-z]+)([^<>]*)>/g;
var a, i;
while ((a = tags.exec(text))) {
    for (i = 0; i < a.length; i += 1) {
        document.writeln(('// [' + i + '] ' + a[i]).entityify());
    }
    document.writeln();
}

I am getting the above JSfiddle warning on both lines with document.writeln().

Foliole answered 24/2, 2013 at 3:1 Comment(2)
Why am I recieving the error 'document.write can be a form of eval'? – Tradesfolk
And more Q/A: [javascript] document.write eval – Tradesfolk
A
18

Note that this is a warning only - but a good one that should be respected. It is actually being generated by a checker called JSLint - and a good read for the reasoning of this warning is available at http://www.jameswiseman.com/blog/2011/03/31/jslint-messages-document-write-can-be-a-form-of-eval/.

Basically, the foundation of this is that "eval is evil" - and that document.write can be used to perform evaluations.

Besides this - and not mentioned in the above, avoid document.write whenever possible, except for maybe simple testing. It writes to the DOM after it is considered to be "complete", and modifications at this point should only be made using the supported DOM methods. Additional details concerning this are covered at Why is document.write considered a "bad practice"? - where it is mentioned that it is "Far better to use the safe and DOM friendly DOM manipulation methods" (document.createElement, element.appendChild, etc.). A good concrete example of this is available at https://developer.mozilla.org/en-US/docs/Web/API/Document_object_model/Using_the_W3C_DOM_Level_1_Core.

Amoroso answered 24/2, 2013 at 3:9 Comment(4)
Every one says this is evil or this is not good. But no one says you may use what instead of document.write. I am new to js, I met this problem, too. I still got no answer how to improve. – Frangipani
@Frangipani - as was mentioned, you should use the supported DOM methods instead. I've further expanded the answer with some additional specifics, including a link to a good concrete example that you may wish to reference. – Amoroso
Oh, sorry, I understand now. I didn't know document.getElementById is called DOM method, which actually has been introduced by my Prof. in the class. 囧 – Frangipani
jameswiseman.com/blog/2011/03/31/… is apt ! – Techno

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