jsp in the js code in the head does not appear

Why is this code in jsp pages inside the body can perform , but on the head can not be executed
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wrapped in a layer of your method code like this:
$ (function () {
/ / middle is your code
});
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on the head of the JS code will be read before the page is loaded , while on the body of the JS code will be read after the entire page has finished loading

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example, you want to call a thing , not loaded, this problem occurs . .

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how to solve it , and put this code into the head , it is also able to perform ?
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with $ (document). ready (function () { js call your method }
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wrapped in a layer of your method code like this:   
$ (function () {   
/ / middle is your code   
});  

positive solution