Iterating over objects in Javascript

Question

I have an obejct of the form:

var test = {"2011":{"10":4,"9":9,"8":15,"7":11,"6":11,"5":13,"4":9,"3":5,"2":9,"1":4,"0":20},"2010":{"11":9,"10":23,"9":58}}

I want to iterate over this so I can hit each year and month combo. I have:

var years = d3.keys(test),
    months = d3.range(12),
    data = [];

So, as a Javascript newbie, to sanity check things I inspect years.

document.writeln(years)
2011,2010

Cool, but when I try to iterate over the years array to pull data out of test the elements I do what I think is correct to test, but the elements are lost:

for(var y in years) {
  document.writeln(y);
}
0,1

How do I iterate over years to get the actual elements (2011,2010) in them?

I see no JSON, which is a text-based data interchange format, here. And please prefer the term "object" over "[associative] array" to avoid dangerous confusion. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Dec 5, 2011 at 17:17
Use console.log or an actual debugger instead of document.write. Also. for $diety's sake, someone write an answer that doesn't incentivize using for-in to loop in an array. — hugomg
– hugomg, Commented Dec 5, 2011 at 17:24
I know it's technically correct to do so, but if I see one more comment like "that's not JSON, that's an object", or something similar, I may just have to stick a knife in something — danimal
– danimal, Commented Jun 17, 2016 at 17:28

Community · Accepted Answer · 2017-05-23 12:08:28Z

2

for in retrieves indices/keys (not the values).

You'll need to modify your loop, like so:

for(var y in years) {
  document.writeln(years[y]);
}

EDIT - per missingno's comment, iterating over an array using for in is a bad idea.

for(var i = 0, l = years.length; i < l; i++) {
  document.writeln(years[i]);
}

A little extra information... if you use a for/in loop against an object it will also iterate over your property keys. For example,

for(var y in test) {
  //This will yield, "2011" and then "2010"
  document.writeln(y);
}

edited May 23, 2017 at 12:08

CommunityBot

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answered Dec 5, 2011 at 17:21

John Strickler

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3 Comments

DrewConway Over a year ago

Is there a way to just access the elements with reference the array, like I would with for-loop over an iterator in Python?

Anurag Over a year ago

@DrewConway what you are looking for is for each for iterating over object property values, but it's non-standard and not widely available. If you want to iterate over arrays, then check out forEach.

John Strickler Over a year ago

I personally recommend underscore.js as a bridge for functions like forEach that aren't widely supported yet.

Alec · Accepted Answer · 2011-12-05 17:22:09Z

2

AFAIK, var y in years will give you the keys to the array years. years[y] should give you the actual value.

answered Dec 5, 2011 at 17:22

Alec

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Comments

IllvilJa · Accepted Answer · 2011-12-05 17:23:17Z

2

y will end up being the index by which you access the elements in years which is 0 and 1 (because years is an array). So for accessing the values of years, try using

writeln(years[y])

instead of

writeln(y)

(IMHO, Javascript is a bit counter intuitive by working this way)

answered Dec 5, 2011 at 17:23

IllvilJa

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Comments

Adam Rackis · Accepted Answer · 2011-12-05 17:44:34Z

2

In javascript for in loops don't do what you think they do; they give you the keys, not the elements

You want

for(var y in years) {
  document.writeln(years[y]);
}

Since you're getting 0,1 back, it looks like years is an actual array, so you could also simply do

for(var i = 0; i < years.length; i++) {
    document.writeln(years[i]);
}

Also, you already edited your title, but a note on terminology. If it's a string, then it's JSON. If it's an object, then, well, it's an object. There's no such thing as a "json object" or "json array"

edited Dec 5, 2011 at 17:44

answered Dec 5, 2011 at 17:21

Adam Rackis

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2 Comments

DrewConway Over a year ago

Thanks for the clarity. I suffer the ignorance of interacting with JSON primary outside of Javascript.

Adam Rackis Over a year ago

@Drew - no worries. I used to call it a json object all the time until someone on SO corrected me :)

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