ECMAScript: SP.ClientContext is undefined or null

Sometime We cannot use SP.ClientContext in Site pages, Web part pages or Application pages. Although We was following this guide from msdn

Setting Up an Application Page for ECMAScript

If so, please replace the below lines

<script type="text/ecmascript" src="/_layouts/SP.Core.js" />
<script type="text/ecmascript" src="/_layouts/SP.Debug.js" />
<script type="text/ecmascript" src="/_layouts/SP.Runtime.Debug.js" />

With

<SharePoint:ScriptLink ID="ScriptLink1" Name="sp.debug.js" LoadAfterUI="true" Localizable="false" runat="server" />

Reference MSDN