String Extention

(function solve() {
  String.prototype.ensureStart = function (str) {
    if (!this.toString().startsWith(str)) { return str + this.toString(); }
    return this.toString();
  };

  String.prototype.ensureEnd = function (str) {
    if (!this.toString().endsWith(str)) { return this.toString() + str; }
    return this.toString();
  };

  String.prototype.isEmpty = function () { return this.toString().length === 0; };

  String.prototype.truncate = function (n) {
    let text = this.toString();
    if (text.length <= n) { return text; }
    else if (n < 4) { return '.'.repeat(n); }
    else {
      text = text.substring(0, n);
      if (text.includes(' ')) {
        let found = true;
        let newtextEnd = undefined;
        while (found) {
          let lastWhiteSpaceIndex = text.lastIndexOf(' ');
          const truncText = `${text.substring(0, lastWhiteSpaceIndex)}...`;
          if (truncText.length <= n) { found = false; newtextEnd = truncText; }
          else { text = text.substring(0, lastWhiteSpaceIndex); }
        }
        return newtextEnd;
      } else { const newtext = text.substring(0, n - 3); return `${newtext}...`; }
    }
  };

  String.format = function (string, ...args) {
    for (let i = 0; i < args.length; i++) {
      if (string.includes(`${i}`)) {
        string = string.replace(`{${i}}`, args[i]);
      }
    }
    return string;
  };
})();