[Vuejs]-Comparing two arrays and pushing the result to new array

You need to drop the this. when referencing groupID and homeGroups as they are not member variables. You can learn more about this here.

Your code will end up looking like this.

compare: function(groupId, homeGroups) {
                groupId.forEach((e1)=>homeGroups.foreach((e2)=>{
                    if(e1 === e2){
                        this.sameId.push(e1)
                    }
                }
            ));
}