Change SPGroup Owner using JSOM

I'm trying to change group owner of a Group by adding another SP group, here is code:

var sourceSpGroup = groupCollection.getByName(groupName1);

var toAddGroup = groupCollection.getByName('SPGroupToAdd');

clientContext.load(sourceSpGroup);
clientContext.load(toAddGroupp);

clientContext.executeQueryAsync(Function.createDelegate(this, this.GetCustGroupDetailsSuccess), Function.createDelegate(this, this.GetCustGroupDetailsFailed));

function GetCustGroupDetailsSuccess() {

    sourceSpGroup.set_owner() = toAddGroupp;

}

However I am getting the error 'Cannot assign to a function result' in console in the line

sourceSpGroup.set_owner() = toAddGroupp;

I could not find any example in MSDN for this property of SP.Group.Owner hence any help would be appreciated.

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