初学typescript

namespace Validation {     export interface StringValidator {         isAcceptable(s: string): boolean;     }     const numberRegexp = /^[0-9]+$/;     export class ZipCodeValidator implements StringValidator {         isAcceptable(s: string) {             return s.length === 5 && numberRegexp.test(s);         }     } } //这句什么意思 let validators: { [s: string]: Validation.StringValidator; } = {}; //这里的中括号什么意思？ validators["ZIP code"] = new Validation.ZipCodeValidator();