Prototype object when looping through array?

Question

I'm getting confusing results when looping through an array.

Filling the array looks like

var tables = [];
// ...
// t is table as jQuery object
tables[t.attr('id')] = t;

Later, when looping through the tables array, I'm getting one more element than I've actually added. The program breakes when the other object reaches t.removeClass()

for (t in tables) {
    var t = tables[t];
    t.removeClass(...);
}

Visual Studio Debugger describes the other object as "clone", which is the first method of the prototype object/property(?).

tables
    [prototype]
        [Methods]
            clone
            ...
        [prototype]
    MyTable0
    MyTable1

I've read that every javascript object comes with a prototype property but why is prototype here treated as an object?

Can you show the code that populates the array?

Tim
– Tim

2013-10-21 07:34:17 +00:00
Commented Oct 21, 2013 at 7:34 — Tim
– Tim, Commented Oct 21, 2013 at 7:34

Denys Séguret · Accepted Answer · 2013-10-21 07:38:20Z

1

Note : if your id aren't numbers, then you don't need an array, look at the other answer.

To loop over an array, don't use the for..in construct.

Use

for (var i=0; i<tables.length; i++) {
    var t = tables[i];
    t.removeClass(...);
}

or if you don't care for IE8 :

tables.forEach(function(t) {
    t.removeClass(...);
});

or with jQuery :

$.each(tables, function(_,t){
    t.removeClass(...);
});

Side note : it looks like somebody poorly enriched the prototype of the native Array class. It's generally seen as bad practice. Doing so and making the property enumerable is even worse. You should consider dropping or fixing the library you use.

edited Oct 21, 2013 at 7:38

answered Oct 21, 2013 at 7:33

Denys Séguret

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2 Comments

Explicat Over a year ago

tables.length is 0 since it's an associative array. tables.forEach doesn't work either.

Denys Séguret Over a year ago

@user2558051 then you shouldn't use an array at all. See eggward's answer.

eggward · Accepted Answer · 2013-10-21 07:39:43Z

0

Your declaration for tables should be:

var tables = {};

answered Oct 21, 2013 at 7:39

eggward

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2 Comments

Denys Séguret Over a year ago

It looks like OP should use an object as map instead of an array, indeed. If you explain it a little, you'll have my +1.

eggward Over a year ago

Because what you want to do is associate t.attr('id') to your t object. Arrays do not behave like that.

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