Angular 4 deep copy for nested object

Question

Here is my code

var destObj= Object.assign({}, sourceObj);
    destObj.fields.forEach(item => {
        if (item.fieldType == 'MULTI_SELECT_LOV') {
            let newValues = [];
            item.selectedValue.forEach(val => {
                newValues.push(val.dataName);
            });
            item.selectedValue = newValues;
        }
    })

after completion of code execution the both 'sourceObj' and 'destObj' are equal.

My intention is do not effect changes which are made on 'destObj' should not reflect in 'sourceObj'.