How to use javascript filter function

Question

I am running the code below in chrome's console. I would like filteredArr to contain 3 objects however I am getting the empty {} as a fourth. The idea is to filter through the flags array and return objects with key == true(or === true - not sure which one works) and the values of true keys append to filteredArr. Any help is appreciated!

var flags = [ { usa: ["red", "white", "blue"] }, null, { germany: ["red", "yellow", "black"] }, {}, { jamaica: ["green", "yellow", "black"] }, "", undefined];

var filteredArr = flags.filter(function(val) {

    return !(val === "" || typeof val == "undefined" || val === null || typeof val == "null");
});

filteredArr;

typeof val == "null" can be removed since typeof null === "object" — SimpleJ
– SimpleJ, Commented Aug 14, 2014 at 21:35
Is this what you need .filter(function(val){return !!val && Object.keys(val).length}) — PSL
– PSL, Commented Aug 14, 2014 at 21:36

Benjamin Gruenbaum · Accepted Answer · 2014-08-14 21:55:38Z

2

First of all, if you want to filter falsy bits, filter on Boolean:

var filteredArr = flags.filter(Boolean)

Which produces the same result as your code but more elegantly. Now if you want to filter objects without enumerable keys you can use Object.keys:

var filteredArr = flags.filter(Boolean).
                        filter(function(el){ return Object.keys(el).length })

Alternatively:

var filteredArr = flags.map(Object).
                        filter(function(x){ return Object.keys(x).length })

Or, flattened into a single filter:

var filtered =  flags.filter(function(x){ return Object.keys(Object(x)).length })

edited Aug 14, 2014 at 21:55

answered Aug 14, 2014 at 21:38

Benjamin Gruenbaum

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8 Comments

elclanrs Over a year ago

Ah, beat me by one sec.

PSL Over a year ago

I know its trivial in this example, but each filter is going to loop through the list right? basically twice...

Benjamin Gruenbaum Over a year ago

@elclanrs Sorry, happens to me all the time too. Thanks for the edit :)

Benjamin Gruenbaum Over a year ago

@PSL Yes. Though in terms of performance, it would be advisable to use a for loop rather than .filter since no engines will optimize that closure away which is expensive here. In terms of iteration count - with the current implementation it will iterate the list twice, however in future versions there will be ways around it (cast into a generator - then chain to generators, effectively iterating the array zero times and performing no actions until it is iterated).

PSL Over a year ago

@BenjaminGruenbaum that will be awesome.. :)

|

tom.alexander · Accepted Answer · 2014-08-14 21:37:41Z

0

An empty object is not null. You'll need to check if an object is empty via something like this:

Object.keys(obj).length === 0

answered Aug 14, 2014 at 21:37

tom.alexander

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