Can IE8 Run JavaScript As IE7?
One feature of Internet Explorer 8 that web developers have come to enjoy is its ability to switch back and forth between IE7 and IE8 rendering standards. However, there are misconceptions about how IE8's compatibility modes handle JavaScript.
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The JScript Engine
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While most popular browsers support JavaScript, Internet Explorer uses Microsoft's own version of ECMAScript known as JScript. IE8 launched with support for a new version of JScript that included changes to make it more standards compliant. This made it easier for programmers to write cross-browser code, but the differences between IE8's version 5.8 of JScript and IE7's version 5.7 introduced problems for programmers wanting to make their code work in both versions.
Internet Explorer Document Modes
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The solution Microsoft devised was to offer multiple document modes in IE8. Before rendering a page, IE8 first checks the document mode defined either by the web page or the browser. The document mode determines the rules that the browser follows. If the document mode is "IE7 Standards," for example, it emulates IE7's rendering engine and JScript engine.
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Defining a Page's Document Mode
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A web developer may choose to define an appropriate document mode within the page itself by adding a meta tag like the following:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" >
Debugging JavaScript Within IE8
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Developers may also change the document type to test different versions of JScript interpretation by using the developer tools in IE8. Pressing F12 in IE8 will open up the developer tools and the document mode may be changed through the appropriate drop-down box.
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References
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