How to hide a part of navigation for a set of users on SharePoint 2013?

First organize all the site users into different groups,

Then go to Site Settings > Navigation and edit the part of navigation you want to hide and set the target audience. 

How to hide left navigation on a particular page in SharePoint 2013?

Add a Script Editor web-part on the page where you need to hide the left navigation and add the following:

<style> 
#sideNavBox
{ 
display: none; 
}
</style>

To re-use this on multiple pages, in case of Publishing site, add the above on Page Layout.

How to prepare basic variables for SharePoint hosted App?

    //Global variables

    var context;

    var hostweburl;

    var appweburl;

    var appContextSite;

    var list;

    var web;

    //Code included inside $( document ).ready() will only run once the page

    //Document Object Model (DOM) is ready for JavaScript code to execute

    $(document).ready(function () {

        //Ensures that the specified file that contains the specified function is loaded and

        //then runs the specified callback function.

        //SP.SOD.executeFunc('file name','function name',methos to call post execution);

        SP.SOD.executeFunc('sp.js', 'SP.ClientContext', getUrl);

    });

    function getUrl() {

        //Fetch query string parameters

        hostweburl = getQueryStringParameter("SPHostUrl");

        appweburl = getQueryStringParameter("SPAppWebUrl");

        //Decode

        hostweburl = decodeURIComponent(hostweburl);

        appweburl = decodeURIComponent(appweburl);

        //Build absolute path to the layouts root with the spHostUrl

        var scriptbase = hostweburl + "/_layouts/15/"; 

        //Before you get SP.js, you have to get SP.Runtime.js

        //Once SP.js can be retrieved, it makes a call to the execOperation method.

        //SP.ProxyWebRequestExecutorFactory is located in sp.requestexecutor.js,

        //which also provides objects, methods, and properties for

        //managing Web request executors for client requests.

        $.getScript(scriptbase + "SP.Runtime.js",

            function () {

                $.getScript(scriptbase + "SP.js",

                function () { $.getScript(scriptbase + "SP.RequestExecutor.js", execCrossDomainRequest); }

                );

            }

        );

            

    }

    function execCrossDomainRequest() {

        context = new SP.ClientContext(appweburl);

        var factory = new SP.ProxyWebRequestExecutorFactory(appweburl);

        context.set_webRequestExecutorFactory(factory);

        appContextSite = new SP.AppContextSite(context, hostweburl);

        web = appContextSite.get_web();

        context.load(web);

        list = web.get_lists().getByTitle("Shared Documents");

        context.load(list);

        context.executeQueryAsync(onSuccess, onFail);

        }

        function onSuccess() {

              alert("List loaded Successfully");   

        }

        // This function is executed if the above call fails

        function onFail(sender, args) {

               alert( args.get_message());

        }

        //Generic function to fetch query sting parameters   

        function getQueryStringParameter(paramToRetrieve)

        {

            var params =

                document.URL.split("?")[1].split("&");

            for (var i = 0; i < params.length; i = i + 1)

            {

                var singleParam = params[i].split("=");

                if (singleParam[0] == paramToRetrieve)

                    return singleParam[1];

            }

        }