javascript - Object.defineProperty or .prototype?

Question

I've seen two different techniques of implementing non-native features in javascript, First is:

if (!String.prototype.startsWith) {
    Object.defineProperty(String.prototype, 'startsWith', {
        enumerable: false,
        configurable: false,
        writable: false,
        value: function(searchString, position) {
            position = position || 0;
            return this.lastIndexOf(searchString, position) === position;
        }
    });
}

and Second is:

String.prototype.startsWith = function(searchString, position) {
    position = position || 0;
    return this.lastIndexOf(searchString, position) === position;
}

I know that the second one is used to attach any method to the prototype chain of a particular standard built-in objects, but first technique is new to me. Can anybody explain what is the difference between them, why one is used and why one not and what are their significances.

Answer 1

In two cases you are adding a new property 'startsWith' in String.prototype.

First differs from the second in this cases:

You can configure the property to be enumerable, writable and configurable.

Writable - true means that you can change its value by assigning any value. If false - you can't change the value

Object.defineProperty(String.prototype, 'startsWith', {
        enumerable: false,
        configurable: false,
        writable: false, // Set to False
        value: function(searchString, position) {
            position = position || 0;
            return this.lastIndexOf(searchString, position) === position;
        }
    });

var test = new String('Test');

test.startsWith = 'New Value';
console.log(test.startsWith); // It still have the previous value in non strict mode