Set Default Value to a Lookup Attribute using JavaScript

Below Example will set the Default Primary Contact ID for Account Entity.

if(crmForm.all.primarycontactid.DataValue == null)
{       
        //Create an array to set as the DataValue for the lookup control.
        var lookupData = new Array();

       //Create an Object add to the array.
        var lookupItem= new Object(); 

       //Set the id, typename, and name properties to the object.
        lookupItem.id = 'SPECIFY GUID of an Entity';
        lookupItem.typename = 'SPECIFY Entity Name';
        lookupItem.name = 'SPECIFY Attribute DISPLAY Value'; 

       // Add the object to the array.
          lookupData[0] = lookupItem; 

      // Set the value of the lookup field to the value of the array.
        crmForm.all.primarycontactid.DataValue = lookupData; 

}