Checking length of dictionary object

Question

I'm trying to check the length here. Tried count. Is there something I'm missing?

var dNames = {}; 
dNames = GetAllNames();

for (var i = 0, l = dName.length; i < l; i++) 
{
        alert("Name: " + dName[i].name);
}

dNames holds name/value pairs. I know that dNames has values in that object but it's still completely skipping over that and when I alert out even dName.length obviously that's not how to do this...so not sure. Looked it up on the web. Could not find anything on this.

There's no "length" property on plain objects. That's something that Array instances have. — Pointy, Jul 26 '10 at 17:42
Ok, anyone know the proper or polite way to award the answer? I know sberry came up with really "the answer" but meder was able to explain it to an intermediate JS guy like myself. I dno't want to piss anyone off. — PositiveGuy, Jul 26 '10 at 18:31
You award the answer to the person who's answer worked for you. — KareemElashmawy, Aug 23 '17 at 00:53

score 679 · Answer 1 · answered Jan 31 '13 at 13:17

679

What I do is use Object.keys() to return a list of all the keys and then get the length of that

Object.keys(dictionary).length

answered Jan 31 '13 at 13:17

Greg Hornby

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Just what I needed! Cheers – Gesias Mar 30 '13 at 07:00
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FYI for the others still supporting IE8: No dice. [link](http://kangax.github.io/es5-compat-table/#Object.keys) – JR MacDonald Apr 11 '13 at 17:35
In Chrome 53, Im seeing Object.keys(dictionary) report a length of 1 even though its an empty dictionary. It has an entry called '$type'. – user99999991 Sep 27 '16 at 16:57
@user999999928 are you possibly using a framework that extends objects somehow? – Greg Hornby Jan 04 '17 at 08:24
4

the most used solution and probably the worst one, you have to loop through the whole dictionary to get the length so it's O(n) instead of O(1) like it should be, ugh – RenaissanceProgrammer Mar 21 '18 at 16:24
Javascript needs a `elegant {}` construct. Inside we can write: `len()`, `for x in {}`, `.items()`, `.keys()`, `.values()`. – Nils Lindemann Apr 21 '19 at 10:09
@RenaissanceProgrammer Unless you have an O(1) alternative, it is also the best one (depending on keys array allocation efficiency). – NetMage Jul 23 '19 at 19:48
2

Now that javascript has the Map object, probably better off using that and using new Map().size – Greg Hornby Jul 24 '19 at 23:36
verbose enough compared to other languages... – Johnny Rockex Jun 08 '21 at 12:13

score 10 · Answer 2 · edited Jul 26 '10 at 17:48

10

var c = {'a':'A', 'b':'B', 'c':'C'};
var count = 0;
for (var i in c) {
   if (c.hasOwnProperty(i)) count++;
}

alert(count);

edited Jul 26 '10 at 17:48

Ben Zotto

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answered Jul 26 '10 at 17:38

sberry

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ok so you have to iterate through...no way to do it off a custom object that has name/values? – PositiveGuy Jul 26 '10 at 17:39
2

I don't **think** there is. Make sure to check the "hasOwnProperty" to stop iteration down up the prototype chain and only check properties. – sberry Jul 26 '10 at 17:41
Thanks..I'm more an OOP guy, but starting to get used to JS more. – PositiveGuy Jul 26 '10 at 17:45
This is the most reliable way to do this in JS. See sberry's note about `hasOwnProperty` as well. The key, @coffeeaddict, is that a JS object can behave like, but is not fundamentally the same as, a "collection" object in other languages. Hence the puzzling lack of a convenience method for getting the total number of properties on it. – Ben Zotto Jul 26 '10 at 17:48

meder omuraliev · Answer 3 · 2010-07-26T17:51:27.563

3

This question is confusing. A regular object, {} doesn't have a length property unless you're intending to make your own function constructor which generates custom objects which do have it ( in which case you didn't specify ).

Meaning, you have to get the "length" by a for..in statement on the object, since length is not set, and increment a counter.

I'm confused as to why you need the length. Are you manually setting 0 on the object, or are you relying on custom string keys? eg obj['foo'] = 'bar';. If the latter, again, why the need for length?

Edit #1: Why can't you just do this?

list = [ {name:'john'}, {name:'bob'} ];

Then iterate over list? The length is already set.

edited Jul 26 '10 at 17:51

answered Jul 26 '10 at 17:44

meder omuraliev

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So an array has a count property...because probably that for is built into the JS native type? – PositiveGuy Jul 26 '10 at 17:45
4

`[]` has `length`, `{}` doesnt. use a `for` loop with `length` for `[]` and a `for..in` for `{}`. – meder omuraliev Jul 26 '10 at 17:47
2

To answer your question yea, it's confusing because I'm not a JS guy, I'm a C# guy. So I needed to check the length of my dictionary to test whether it had any objects in it. For regular error handling I guess you'd just check for null on a dictionary but just wanted to see how many returned and was added to my dictionary. So the problem was not how to iterate, just see how to get the count which I just thought you could with a dictionary type (nvp) object so-to-say. – PositiveGuy Jul 26 '10 at 18:27
yes, I'm using obj['foo'] = 'bar'; type of deal to get something that acts as a dictionary. – PositiveGuy Jul 26 '10 at 18:28

score 1 · Answer 4 · answered Aug 02 '12 at 14:38

Count and show keys in a dictionary (run in console):

o=[];count=0; for (i in topicNames) { ++count; o.push(count+": "+ i) } o.join("\n")

Sample output:

"1: Phase-out Left-hand
2: Define All Top Level Taxonomies But Processes
3: 987
4: 16:00
5: Identify suppliers"

Simple count function:

function size_dict(d){c=0; for (i in d) ++c; return c}

Checking length of dictionary object

4 Answers4