Creating Client Context using JavaScript

There are two ways:

1. For the current site.

var clientContext = new SP.ClientContext.get_current();

2. For a specific site.

var clientContext = new SP.ClientContext('https://sharepointsite');

If get_current() throws undefined error then load "sp.js" explicitly. see below code:

SP.SOD.executeFunc('sp.js', null, createContextMethod);

createContextMethod(){
   var clientContext = new SP.ClientContext.get_current();
}