What am I doing wrong in my recursive javascript function?

Question

I am trying to create a recursive function that filters an object, depending on the properties in the second passed argument.

Filtering works well, but my arrays are being replaced with empty objects. What could I do different so that this doesn’t occur?

var filter = function(obj, fields) {
 var key, o;
 if (typeof obj !== "object") { return obj;}
 o = {};
 for (key in obj) {
 if (fields[key]) {
 o[key] = filter(obj[key], fields[key]);
 }
 }
 return o;
};
data = {name: "John Doe", followers: [], notAllowedProp: false}
allowedFields = {name: true, followers: true}
console.log(filter(data, allowedFields));
// => Object { name: "John Doe", followers: {}}

• AMember: you are returning an empty object regardless of the fields type I suggest checking the filed type and initiating o by it.

Answer 1

Try this on a console:

> typeof []
"object"

To check for an array more robustly, you can use Object.prototype.toString:

> Object.prototype.toString.call([])
"[object Array]"

Object.prototype.toString has well-defined behavior for all native objects, and is quite reliable. Thus, for example, if you want to return anything that is not a "true" object, you could write:

if (Object.prototype.toString(obj) !== "[object Object]") {
 return obj;
}

• maerics: Or [].constructor === Array.
• voithos: Although they work, there are certain problems that can arise with those techniques that don't occur if you use Object.prototype.toString.
• Peter Olson: @voithos If you're not using iframes that interact with each other programmatically, you're fine using instanceof.
• voithos: @PeterOlson: Sure, but that scenario is a plus for using Object.prototype.toString regardless. And from another standpoint, I'd argue that Object.prototype.toString is more readable than manipulating typeof and instanceof.
• voithos: @pebbl: Yes, what you're speaking of is duck typing, which is fine in most cases.

Answer 2

Arrays have type object. Consequently, you'll need to edit this line if you want to return early for arrays:

if (typeof obj !== "object") { return obj; } 

There are many ways to check for an array, but this is what I would do:

if(typeof obj !== "object" && !(obj instanceof Array)) { return obj; }