05. String Extension

(function () {
  String.prototype.isEmpty = function () {
    return this == '';
  };
  String.prototype.ensureStart = function (str) {
    if (this.startsWith(str)) return this.toString();
    return str + this.toString();
  };
  String.prototype.ensureEnd = function (str) {
    if (this.endsWith(str)) return this.toString();
    return this.toString() + str;
  };
  String.prototype.truncate = function (n) {
    if (this.length < 4) return '.'.repeat(n);
    if (this.length <= n) return this.toString();
    if (this.indexOf(' ') == -1) {
      return `${this.slice(0, n - 3)}...`;
    }
    const tokens = this.split(' ');
    let result = tokens[0];
    for (let i = 1; i < tokens.length; i++) {
      if (result.length + tokens[i].length + 4 > n) // space + '...'
      { return `${result}...`; }
      result += ` ${tokens[i]}`;
    }
  };
  String.format = function (str) {
    for (let i = 1; i < arguments.length; i++) str = str.replace(`{${i - 1}}`, arguments[i]);
    return str;
  };
}());