JS array - weird values with "for in" loop

Question

Have JS array w/o keys

code 1

var psnb_arr=['14-007', '14-008', '14-009'];
console.log(psnb_arr);

//output in firebug: ["14-007", "14-008", "14-009"]

code 2

for (var i in psnb_arr) {
  var psnb=psnb_arr[i];
  console.log(i+'-'+psnb);
}

//wierd output in firebug:
0-14-007
1-14-008
2-14-009

copy-function (start,length){start=start||0;if(start<0)start=this.length+start;

length=length||(this.length-start);var newArray=[];for(var i=0;i<length;i++)newArray[i]=this[start++];return newArray;}

remove-function (item){var i=0;var len=this.length;while(i<len){if(this[i]===item){this.splice(i,1);len--;}else{i++;}} return this;}

contains-function (item,from){return this.indexOf(item,from)!=-1;}


...

what is this wierd output in firebug for console.log within the loop? I mean the text that is coming right after "2-14-009" - "copy-function..."

i = the "0-" on the first output, just as it's written in your code. — Diodeus - James MacFarlane
– Diodeus - James MacFarlane, Commented Feb 14, 2014 at 21:00
Your "object without keys" is just an array. It does have keys, they are numeric. — gen_Eric
– gen_Eric, Commented Feb 14, 2014 at 21:01
I mean the text that is coming right after "2-14-009" - "copy-function..." — ihtus
– ihtus, Commented Feb 14, 2014 at 21:01
Can you make a Fiddle to reproduce your problem? You dont have this in another browser? I just get the array output, not the other stuff. What version Firefox / firebug are you using? — putvande
– putvande, Commented Feb 14, 2014 at 21:02

gen_Eric · Accepted Answer · 2014-02-14 21:09:13Z

2

You do not have an object, nor do you have an associative array (these no not exist in JavaScript). What you have in a "normal" (numeric) array. It does have keys; the array indexes (that's where the 0-, etc. are coming from).

The problem you are seeing is because you are using for...in on an array. This is a bad practice. The Array prototype can contain properties (sometimes JavaScript libraries add these), so you are iterating over all of those properties, not just the array values.

You want to use a normal for loop here.

for(var i = 0; i < psnb_arr.length; i++){
    var psnb=psnb_arr[i];
    console.log(i+'-'+psnb);
}

edited Feb 14, 2014 at 21:09

answered Feb 14, 2014 at 21:04

gen_Eric

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7 Comments

gen_Eric Over a year ago

@putvande: Yeah, I'm fully aware. I explained exactly why that's happening and how to fix it. What's the problem?

ihtus Over a year ago

thank you, I didn't know about "sometimes JavaScript libraries add these"

gen_Eric Over a year ago

@ihtus: Are you using something like Prototype or an ES5 shim? That's normally where this comes from. They add methods to Array.prototype.

Eugene P. Over a year ago

as another option hasOwnProperty can fix the issue above, but anyway @RocketHazmat explanation is OK

gen_Eric Over a year ago

@EugeneP: Yes, you can add hasOwnProperty to the for..in loop, but it's easier (and I think better practice) to just use a normal for loop (or .forEach()). :-)

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