Modifying array prototype causes foreach to behave differently

Question

I noticed that whenever i modify an array prototype it changes the behavior of foreachs. I have created the example below to show you guys what im doing:

Array.prototype.x = 10;
for(var i in [1,2,3]){
    alert(i);
}

This example shows four alerts: "1", "2", "3" and "x";

Why this code give me 4 alerts?

How can i modify array prototype without interfere with the foreach ?

Of course! You add one more enumerable property to all array instances. Why are you surprised? — dfsq
– dfsq, Commented Jul 12, 2014 at 14:50
It is generally considered a bad idea to use a for/in loop for arrays. Stick to using for/in with objects, and a regular for loop for arrays. However, you can extend Array.prototype without making the extensions enumerable; see bfontaine's correct answer below. — Ethan Brown
– Ethan Brown, Commented Jul 12, 2014 at 14:53

bfontaine · Accepted Answer · 2014-07-12 14:51:31Z

2

You need to define your property as “non-enumerable”:

Object.defineProperty(Array.prototype, 'x', {
    enumerable: false,
    value: 10
});

Here is a JSFiddle that shows the code in action.

answered Jul 12, 2014 at 14:51

bfontaine

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