5.String extension

(function stringExtension() {
    let str = 'my string';

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

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

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

    String.prototype.truncate = function (n) {
        if (this.toString().length > n) {

            let str=this.substring(0,n-1);
            let spaceIndex=str.lastIndexOf(' ');
            str = str.substring(0,spaceIndex) + '...';
            return str
        }

        if(n<4) return '.'.repeat(n);
        return this.toString();
    };

    String.format = function (string, ...params) {
        for (let i = 0; i < params.length; i++) {
            let index = string.indexOf("{" + i + "}");
            while (index != -1) {
                string = string.replace("{" + i + "}", params[i]);
                index = string.indexOf("{" + i + "}");
            }
        }
        return string;
    };

})();