ES6

Flags.js

function getFlags(re) {
    var text = re.toString();
    return text.substring(text.lastIndexOf("/") + 1, text.length);
}
// toString() is "/ab/g"
var re = /ab/g;
console.log(getFlags(re));          // "g"

let re = /ab/g;
             console.log(re.source);     // "ab"
             console.log(re.flags);      // "g"