JavaScript Getters and Setters issue

Question

Here's the code I'm working with at the moment:

Object.defineProperty(String.prototype, "testy", {
    get: function() {
        return this.string;
    },
    set: function(string) {
        this.string = string;
    }
});

console.log("tessfef3t".testy());

Before I would've used String.prototype.testy = function {} , however I've been told that using something similar to the the code above is the better way to do it. I'm not sure how that is meant to work but I haven't got that code to work yet.

Could someone show me how to correctly do what I'm doing?

Thanks

Answer 1

testy is kind of a "fake" property -- it has no value of its own, but setting or getting its value will invoke its set and get functions. You can use it like:

var foo = new String();

// this invokes the setter and sets foo.string to 5:
foo.testy = 5; 

// this invokes the getter and prints the value of foo.string
console.log(foo.testy); 

// this prints 5, because foo.string has been set by the setter
console.log(foo.string);

EDIT:

Now I see your comment about what you actually want to happen. It's not possible to alter the value of a string object, because strings are immutable. You'd have to destroy the string object and replace it with a new one, which is not possible within the object's own member function.

Answer 2

If all you're doing in your getter and setter is setting a property that is not used elsewhere and there's no other processing then you gain nothing by using a getter and setter so you're better off with your original approach. Furthermore, you won't be able to retrieve a property you set on a string anyway. A string value in JavaScript (which is what a string literal such as "foo" creates, and is the only kind of string that is generally useful) is not an object so attempting to accessing a property requires special handling: effectively a temporary String object is created and thrown away immediately afterwards.

Finally, another major disadvantage of getters and setters is that they are specified in ECMAScript 5 and as such are only available in relatively recent browsers. They aren't implemented in IE < 9, for example.

Answer 3

This worked fine for me:

Object.defineProperty(String.prototype, "byteLength", {
    get: function() {
        var str = "" + this; // Get internal value
        // Compute size in actual bytes vs Unicode characters
        // per http://stackoverflow.com/a/23329386/912236
        for (var b = str.length, c = str.length - 1; 0 <= c; c--) {
            var a = str.charCodeAt(c);
            127 < a && 2047 >= a ? b++ : 2047 < a && 65535 >= a && (b += 2);
            56320 <= a && 57343 >= a && c--;
        }
        return b;
    }
});

> "Hello".byteLength
< 5