CODEWITHSUNDEEP
javascriptprototype
telex-wap
telex-wap

Reputation: 852

This JS function works well, but when adding as a new String method with prototype, fails...

I have this code, and a bit of a beginner's problem:

function findfruit (food){

var fruitvalues = ["apple", "orange", "banana", "mango", "kiwi", "watermelon"];
if (fruitsvalues.indexOf(food) != -1){
    return true;
    }
else {
    return false;
    }
    }


var food = prompt("Write down any food you like");
findfruit(food) // returns always the correct answer, true or false

So here is the thing: This works well without problems, but if I do: 

String.prototype.fruit = findfruit;

and then I try to make

if (food.fruit != true) {
    alert("your food is not a fruit");
    }
else {
    alert("your food is a fruit");

it doesn't work, and it always results "false". I suspect that my problem is that when I invoke food.fruit, the function findfruitstarts without using the string variable foodthat I just created, but I don't understand why. I am just reading tutorials as for how to use prototype, and I cannot find anything wrong (although I am sure there is something).

I would appreciate any help. I know this must be pretty easy, but the other questions about prototype that I found here couldn't help.

Thanks!

Upvotes: 0

Views: 55

Answers (2)

lqc
lqc

Reputation: 7328

1) Writing food.fruit is not a function invocation. It will just return the function itself (which obviously can't be equal to true).

2) You can access the object from which the function is invoked as this. It won't be passed as an argument to your function (this isn't Python). The correct implementation is

String.prototype.fruit = function() {
   var fruitvalues = ["apple", "orange", "banana"];
   return (fruitvalues.indexOf(this.toString()) != -1);
}

3) Adding functions like this to prototypes of built-in types is discouraged. It's tempting, but you have to ask yourself: "Is this generic enough that I will need it everywhere in my program ?".

Upvotes: 1

Ryan
Ryan

Reputation: 288

If you simply use this to reference the string, you'll actually be scoped to the function. this.valueOf will look to the parent object (String).

String.prototype.fruit = function () {
var fruitvalues = ["apple", "orange", "banana", "mango", "kiwi", "watermelon"];
if (fruitvalues.indexOf(this.valueOf()) != -1){
    return true;
}
else {
    return false;
}
}

var food = 'apple';

if (food.fruit() != true) {
    console.log("your food is not a fruit");
}
else {
    console.log("your food is a fruit");
}

Upvotes: 0

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