const nameAgeMap: { [index: string]: number } = {};
nameAgeMap.Jack = 25; // no error
nameAgeMap.Mark = "Fifty"; // Error: Type 'string' is not assignable to type 'number'.
console.log(nameAgeMap);
prog.ts(3,7): error TS2322: Type 'string' is not assignable to type 'number'.