javascript for loop

Question

One of my co-workers created a silly bug in mongo mapreduce, he should have created an object but actually passed the string "{}". The weird thing was the script hasn't crashed but actually iterated over the string, so I did some investigation and found the following:

The script x="bla"; for (i in x) { alert(i); }; works the following way with different browsers:

IE(8): - (no alerts)
Chrome: 0 1 2
Firefox (an older one): 0 1 2 vBlength
Firefox 5: 0 1 2
MongoDB: 0 1 2 trim ltrim rtrim

I know perfectly well it's not a valid syntax, just pure curiosity, does anybody know why browsers behave the way they do? I'm especially interested in MongoDB's behaviour.

Justin Niessner · Accepted Answer · 2011-07-15 15:19:31Z

5

Actually, that is completely valid syntax.

Using the for...in loop in JavaScript will not only loop over arrays but also properties (i is simply an index, not an actual value in the array).

Check out the documentation at the Mozilla Developer Network:

for...in - MDN Docs

Their example should make things more clear.

answered Jul 15, 2011 at 15:19

Justin Niessner

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3 Comments

Karoly Horvath Over a year ago

So basically I see the extra values because those properties aren't hidden?

Justin Niessner Over a year ago

@yi_H - Pretty much. You're seeing the differences in implementation details of both the for...in loop and the string object in the different browsers.

Simon Over a year ago

See Douglas Crockford's piece of advice about the for statement to avoid bad surprises.

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