Object.defineProperty or .prototype?

Question

I saw two different methods for implementing non-local functions in javascript, First:

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:

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

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

Answer 1

In two cases, you add a new property 'startsWith' to String.prototype.

First different from the second in this case:

enumerable, writable configurable.

Writable - true , , . false -

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

Hide result

Enumerable - true , for in.

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

var test = new String('Test');

for(var key in test){
   console.log(key)  ;
}

Hide result

- true , , .

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

    
    delete String.prototype.startsWith; // It will not delete the property
    console.log(String.prototype.startsWith);

Hide result

, , .