for(wtf in obj)¶

Javascript has a great convenience method for iterating through objects or arrays, called

for(item in container)

Which should be familiar to developers acquainted with Python/Ruby/C++/many other languages, which have similar concepts of iterables.

In [1]:

var obj = {
    a: 1,
    b: 2,
};

Out[1]:

undefined

In [2]:

for (item in obj) {
    console.log("obj has key %s", item);
}

obj has key a
obj has key b

Out[2]:

undefined

But JavaScript always has a way to ruin your day:

In [3]:

Object.prototype.wtf = 'wtf';

for (item in obj) {
    console.log("obj has key %s", item);
}

obj has key a
obj has key b
obj has key wtf

Out[3]:

undefined

So for-in is never safe. There is a helpful Object.keys() function (don't ask me why it's not a method):

In [4]:

Object.keys(obj);

Out[4]:

[ 'a', 'b' ]

So let's give it a try, shall we?

In [6]:

for (var key in Object.keys(obj)) {
    console.log("Object.keys(obj) has key", key);
}

Object.keys(obj) has key 0
Object.keys(obj) has key 1
Object.keys(obj) has key wtf

Out[6]:

undefined

WTF, indeed! for-in arrays iterates through indices for some reason instead of elements, plus since arrays are objects, for-in is also ruined.

You can use Array.map with Object.keys, if it's okay to call a function on every iteration:

In [10]:

Object.keys(obj).map(function (key) {
    console.log("obj[%s] = %s", key, obj[key]);
});

obj[a] = 1
obj[b] = 2

Out[10]:

[ undefined, undefined ]

Or you can resort to the super-awesome Object.hasOwnProperty filter on the original for-in:

In [11]:

for(var key in obj) {
    if (!obj.hasOwnProperty(key)) continue;
    console.log("obj[%s] = %s", key, obj[key]);
}

obj[a] = 1
obj[b] = 2

Out[11]:

undefined

There's allegedly hope in the form of for-of, which aims to do what for-in always should have done, but that is dashed by it apparently being crazy slow.