Map an array of objects using javascript, it returns an undefine values

What is the problem with my code below ? I did map based on the key that i wanted to get but i get an undefined values?. I wanted to get each random code. It did return that there are 6 total random code but it is undefined. Any problem? Thank you.

my code

let result =  me.record.job_detail.questionaires.map(a => a.random_code);

the result

result

Array(6) [ undefined, undefined, undefined, undefined, undefined, undefined ]

array of ojects (me.record.job_detail.questionaires)

[ “{‘created_at’: ‘2019-02-07’, ‘department’: ‘sme’, ‘timelimit’: None, ‘id’: 16, ‘sub_title’: ‘this is an exam’, ‘title’: ‘exam’, ‘random_code’: ’50-49361157339′}”, “{‘created_at’: ‘2019-02-07’, ‘department’: ‘TEest DEP’, ‘timelimit’: None, ‘id’: 10, ‘sub_title’: ‘this is a test’, ‘title’: ‘Hello’, ‘random_code’: ’50-612511266113′}”, “{‘created_at’: ‘2019-02-07’, ‘department’: ‘SYSTECH’, ‘timelimit’: None, ‘id’: 9, ‘sub_title’: ‘Trying to test’, ‘title’: ‘Test 101’, ‘random_code’: ’50-8381637318′}”, “{‘created_at’: ‘2019-02-07’, ‘department’: ‘SME’, ‘timelimit’: None, ‘id’: 8, ‘sub_title’: ‘just to test’, ‘title’: ‘Exam’, ‘random_code’: ’50-6819378387′}”, “{‘created_at’: ‘2019-02-08’, ‘department’: ‘test’, ‘timelimit’: None, ‘id’: 35, ‘sub_title’: ‘test’, ‘title’: ‘test’, ‘random_code’: ’50-2143566173′}”, “{‘created_at’: ‘2019-02-07’, ‘department’: ‘Test’, ‘timelimit’: None, ‘id’: 13, ‘sub_title’: ‘test’, ‘title’: ‘test’, ‘random_code’: ’50-2961010112644′}” ]

Source: AngularJS