Javascript code organization

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ASP.NET

Javascript

jQuery

I just started using jquery lately & developed a small application. However the pagesize was increasing because of a lot of javascript code so I am starting to move the code in separate script files. However,

1) I cannot access other jquery plugins functions when I move my code to separate js file because they are located in other script files. How do I use the other libraries in my separate script file ?

2) Am I doing it the wrong way ? This does not work at all in my separate script file (foo.js). It says stringify is not a function

<script src="../assets/scripts/jquery.json-2.2.min.js" type="text/javascript">
</script>

$(document).ready(function(){
    JSON.stringify('test message');
});