Ans. The ECMAScript library is available in a number of JS files in the LAYOUTS folder. The main file among number of .js files is SP.js. When you include this file in the APSX page using a ScriptLink control, all other required JS files are loaded automatically. By linking SP.js to your page, the SP namespace gets registered. SP is the SharePoint namespace that contains all objects. For debugging purposes every js file also has a 'debug' equivalent in the same folder.