How to get selected records ID from List View using Javascript / Ajax Toolkit in Salesforce?

Create a new Button on Lead of type List Button. Add the button on Lead List View Layout and write below Javascript code:

{!RequireScript("/js/functions.js")}
var recordsSelected = {!GetRecordIds($ObjectType.Lead)}
for(var i=0; i < recordsSelected .length ; i++) {
alert('Selected ID '+recordsSelected[i]);
}