_spPageContextInfo javascript variable in sharepoint

Introduction

_spPageContextInfo is a  JavaScript context variable, which will rendered for each SharePoint page. _spPageContextInfo holds few proprieties which will be useful in JavaScript and client object model code. If you want to know the properties of _spPageContextInfo, go to view page source and find text with "_spPageContextInfo".

When It will be useful:

• When we are making any ajax calls, we might need the server URL dynamically. In this case we can use _spPageContextInfo.siteAbsoluteUrl property. 
• If you want to know the current SharePoint List GUID, use _spPageContextInfo.pageListId.
• Even it has more properties also(which are self explanatory). We can use them according to the requirements.

Sample Code:

	if (_spPageContextInfo != null) {
            if (_spPageContextInfo.pageListId != null) {
                var pageListId = _spPageContextInfo.pageListId;
            }
            if (_spPageContextInfo.siteAbsoluteUrl && _spPageContextInfo.siteAbsoluteUrl != '') {
               var siteAbsoluteUrl = _spPageContextInfo.siteAbsoluteUrl
            }
        }

$.ajax({
            type: "GET",
            url: siteAbsoluteUrl + "/_layouts/15/Test.aspx?listid=" + pageListId + "",
            data: "",
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function (data) {                
                    alert('success');
            },
            error: function () {
                alert('failed ');

            }
        });

_spPageContextInfo Object Properties found for SharePoint 2013 application:

• alertsEnabled: false
• allowSilverlightPrompt :"True"
• clientServerTimeDelta :-8980
• crossDomainPhotosEnabled: false
• currentCultureName: "en-US"
• currentLanguage: 1033
• currentUICultureName: "en-US"
• layoutsUrl: "_layouts/15"
• pageListId:  "{df5e2d45-ad0b-4608-a813-ded6a97511db}"
• pagePersonalizationScope:  1
• serverRequestPath:  "/sites/dotnetmirror/Lists/Calender custom/calendar.aspx"
• siteAbsoluteUrl: "http://computerName:5000/sites/dotnetmirror"
• siteClientTag: "0$$15.0.4433.1506"
• siteServerRelativeUrl; "/sites/dotnetmirror"
• systemUserKey: "i:0).w|s-1-5-21-2086049...90378103-3194450012-500"
• tenantAppVersion: "0"
• userId: 1
• webAbsoluteUrl: "http://computerName:5000/sites/dotnetmirror"
• webLanguage: 1033
• webLogoUrl:"_layouts/15/images/siteicon.png"
• webPermMasks: Object { High=2147483647, Low=4294967295}
• webServerRelativeUrl: "/sites/dotnetmirror"
• webTemplate: "1"
• webTitle : "dotnetmirror"