converting object of arrays to an array of objects in a desired format
Your code is not bad. You use functional style to loop through the data, which is good. But inside the loop you do not have to check with if-elseif-elseif, as this is not solid when it comes to many many cases. 
Maybe it would be better to use a map object to guide the process of matching data.

getSelectedItemsIds = selectedItemsList =&gt; {
    const keyNamesMap = {
        projectIds: "PROJECT",
        subjectGroupIds: "SUBJECT_GROUP",   
        subjectIds: "SUBJECT"
    };
    let selectedItems = [];

    Object.keys(keyNamesMap).map( k =&gt; {
        selectedItemsList[k].map ( id =&gt; {
            selectedItems.push (
                {
                    type: keyNamesMap[k],
                    id: id
                }
            );
        });       
    });
    return selectedItems;
}


This way it is more clear and more easy to add another type.