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Editorial Team
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Asked: 2026-06-04T18:40:13+00:00

Running this code in browser, if no colour is entered, ‘green'(undefined) is rendered. Random/garbage

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Running this code in browser, if no colour is entered, ‘green'(undefined) is rendered.

Random/garbage value leads to black background…
…not selecting or transparent gives white(null) 

<HTML>
<HEAD>
<TITLE>Document Object Demo</TITLE>
</HEAD>
<BODY>
<SCRIPT LANGUAGE=Javascript>
var sAnswer = window.prompt("Please enter your favourite color : choose from the following Yellow, Green, Red, Blue, Pink, Orange, Black, Gray")
document.bgColor = sAnswer
document. write ("<H1>Welcome " + sAnswer + ":-)!!</H1><HR>We are very happy to demonstrate the usage of <I> document. write</I>. Do remember to view the source!<P>")
</SCRIPT>
</BODY>
</HTML>

I use IE9…!

Why is this happening?

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1 Answer

  1. Editorial Team

    You are having this problem because the browser will try to parse something out of w/e you set the bgColor to…

    Now your Javascript is fairly questionable, and I can not think of a reason you’d want to do this, but your code should probably look more like this.

    (function () {
    // Prompts the user to give an answer, this prevents further code execution
    var sAnswer = window.prompt("Please enter your favourite color : choose from the following Yellow, Green, Red, Blue, Pink, Orange, Black, Gray");

    // Takes care of all falsy valuyes, 0, null, undefined, false
    // Ideally some more validation on the input should be done
    if(sAnswer) {
        document.body.style.backgroundColor = sAnswer;
        document.write ("<h1>Welcome " + sAnswer + ":-)!!</h1><hr><p>We are very happy to demonstrate the usage of <i> document. write</i>. Do remember to view the source!</p>");
    } else {
        document.write ("<p>You did not fill in a valid answer! Refresh.</p>");
    }
}());

    OR keep asking the question until a valid answer is given.. (fairly evil imo)

    (function () {
    var sAnswer;

    do {
        sAnswer = window.prompt("Please enter your favourite color : choose from the following Yellow, Green, Red, Blue, Pink, Orange, Black, Gray")
    } while ( !sAnswer );

    document.body.style.backgroundColor = sAnswer;
    document.write ("<h1>Welcome " + sAnswer + ":-)!!</h1><hr><p>We are very happy to demonstrate the usage of <i> document. write</i>. Do remember to view the source!</p>");

}());

    Code not tested but fairly confident it should work :p

    Edit, how does it get to the colour green?

    In short, undefined is typecasted to a string which can be used to set the bgColor property, which in turn is parsed and interpreted as a color, making invalid hex characters count for 0. Read this for a more in depth explanation.

    Internet Explorer used this interpretation and the other browsers followed so to not break backwards compatibility.

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