JavaScript to get page and site information like url,.. in SharePoint

use 

alert(_spPageContextInfo.siteAbsoluteUrl);

_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"