declare global {
interface String {
    format(): string;
}}

String.prototype.format = function () :string {
var result = arguments[0];
for (var i = 0; i < arguments.length - 1; i++) {
    var reg = new RegExp("\{" + i + "\}", "gm");
    result = result.replace(reg, arguments[i + 1]);
}
return result;}

declare global {
    interface String {
        foo(): number;
    }
}

declare global {
    interface StringConstructor {
        format(): string;
    }
}

String.format = function (...args: string[]) {
    ...
}

interface String {
  format(...args: string[]): string;
}

interface String {
  format(...args: string[]): string;
}

String.prototype.format = function (...args: string[]): string {
  var s = this;
  return s.replace(/{(d+)}/g, function (match, number) {
    return (typeof args[number] != 'undefined') ? args[number] : match;
  });
};

import '../../string.extensions';

console.log("Hello {0}".format("world"));