function getFields(list, field, otherwise) {

    //  determine once whether or not to use the 'otherwise'

    var alt = typeof otherwise !== 'undefined';

​

    //  reduce the provided list to an array only containing the requested field

    return list.reduce(function(carry, item) {

        //  If item is an object and contains the field, add its value and the value of 'otherwise' if it was provided

        if (typeof item === 'object' && field in item) {

            carry.push(item[field]);

        }

        else if (alt) {

            carry.push(otherwise);

        }

​

        //  return the 'carry' (which is the list of matched field values)

        return carry;

    }, []);

}

var myArray = [{foo: 1, bar: 2}, {bar: 3, baz: 4}, {foo: 5, baz: 6}];

​

//. no 'otherwise' (meaning it'll be undefined)

console.log(getFields(myArray, 'foo'));

​

//. otherwise=3

console.log(getFields(myArray, 'foo', 3));