Messing With This In Prototype's Method

Ok, here's the situation: var vowels = ['a', 'i', 'y', 'e', 'o', 'u']; String.prototype.isVowel = function () { return vowels.indexOf(this) !== -1; }; alert('a'.isVowel()); It

Solution 1:

The problem is what is called boxing. If you have a primitive value and then call a method on that value, it will be wrapped as an object of the type String, Number, Boolean, or what have you. This is equivalent to calling new String('a'), for instance. (It is documented in the ECMAScript specification.)

new String('a') is not the same as 'a'. It therefore is not found in the array.

What you need to do, therefore, is convert the value back into the primitive value. You can always accomplish this with the valueOf method:

return vowels.indexOf(this.valueOf()) !== -1;

Your method is to call this[0]. This works, because it gets the first character from the string as a primitive, which is then found in the string. The downside is that ab would also be considered a vowel by this method.

---

Similarly with numbers:

Number.prototype.is5 = function () { returnthis.valueOf() === 5; };

---

One other point: this is the default behaviour of Javascript. It is not obvious or intuitive, as you have shown. In ECMAScript 5, "strict mode" was created, where all kinds of undesirable behaviours are disabled. Auto-boxing is one of them. So, if you use a modern browser, this will have the expected behaviour:

Number.prototype.is5 = function () {
    "use strict";
    returnthis === 5;
};

Solution 2:

Look at the way you declared String.prototype.isVowel, which worked acceptably. Now look at Number.prototipe.is5(). See the differences?

• prototipe is misspelled, and should be prototype.

• is5() shouldn't have () after it.

That leaves you with this:

Number.prototype.is5 = //...

You should've gotten an error when setting Number.prototipe.is5(), so maybe these are all just typos in posting.