Select a Radio Button in Ajax Response

I have an ajax response that replaces the contents in a div. The response is this:

<input onchange="switch(this.value)" value="val" id="code" checked="checked" type="radio" name="code" />

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The radio button shows up just fine, however, it is not checked. Any ideas why?

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It's a radio button, not a checkbox and maybe another radio button in the same group (i.e. with the same 'name') is already checked.

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There is no need to write any code for onclick. The default behavior of radio button is to change its state on click. Also value='val' attribute. I guess it's messing up with checked attribute.