// Say what the thing is
interface MyThingy {
  [key: string]: string|number|Date
}
// Include that when declaring the variable
var datasource: MyThingy[] = [
  { name: 'x', age: 20, address: 'yyyy' },
  { name: 's', age: 30, address: 'zzzz' }
];

let keys: Object[] = Object.keys(datasource[0]);
let dlen: number = datasource.length as number;
while (dlen--) {
    let kLn: number = keys.length;
    while (kLn--) {
        let key: string = keys[kLn] as string;
        let mkey: string | number | Date = datasource[dlen][key] as number;
    }
}