XMPMeta is undefined

Following is the code snippet :

if (ExternalObject.AdobeXMPScript == undefined) {     
    ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript"); 
} 

var registeredNameSpace = XMPMeta.registerNamespace(context.destNamespace, context.nodeName);

I am getting error "XMPMeta is undefined".

The error is coming on MacOS but the code is working fine on Windows.