Thursday 23 December 2010

Say what?

(Updated 2014/12/30)

Something that sometimes comes up in JavaScript work is the need to figure out what kind of thing you're dealing with — is it a string? a number? a Date? And JavaScript has various features to help you do that, but there are some "gotchas" to watch out for. In this post I'll talk about various strategies for figuring out what things are.

Basically, you have four tools you can use, each of which has its place:

  • typeof
  • instanceof
  • Object.prototype.toString
  • Learn to stop worrying and love the duck

Let's look at each of them:


typeof is fast but fairly limited. It basically tells you whether something is a primitive or an object, and if it's a primitive it tells you what kind of primitive it is, but for all objects other than functions, it just says "object". So:

console.log(typeof "testing"); // "string"
console.log(typeof {}); // "object"

But it's useful for, say, determining if something is a function:

if (typeof x === "function") {
// Yes it is

(Although in some environments, host-provided functions like alert give us "object" rather than "function".)

typeof's chief advantage is speed, although with modern JavaScript engines, speed isn't anywhere near the concern it was a few years ago.


instanceof sort of picks up where typeof leaves off. It's good for checking whether something is a specific kind of object, e.g.:

var dt = new Date(...); 
console.log(dt instanceof Date); // true

Specifically, obj instanceof Func it looks at each object in obj's prototype chain and sees if any of them is the one that Func assigns when used to construct objects. In the example above, for instance, since dt's prototype is Date.prototype, dt instanceof Date is true. dt instanceof Object is also true, because dt's prototype's prototype is Object.prototype. If you'd set up a multi-level hierarchy of constructors and prototypes so that Cat derives from Mammal which derives from Animal, and used c = new Cat(), instanceof would be true for a c instanceof Cat, c instanceof Mammal, and c instanceof Animal (and c instanceof Object, normally).

You do have to be careful with instanceof in some edge cases, though, particularly if you're writing a library and so have less control / knowledge of the environment in which it will be running. The issue is that if you're working in a multiple-window environment (frames, iframes), you might receive a Date d (for instance) from another window, in which case d instanceof Date will be false — because d's prototype is the Date.prototype in the other window, not the Date.prototype in the window where your code is running. And in most cases you don't care, you just want to know whether it has all the Date stuff on it so you can use it.


Calling Object.prototype.toString is slower than typeof or instanceof, but more useful for differentiating what kind of object you have — but sadly, only if the object is one created by one of the built-in JavaScript constructor functions like Date or String, not our own constructor functions. The return value of the Object prototype's toString function is defined in the standard for all the built-in constructor functions (Array, Date, etc.): It returns "[object ___]" where ___ is the constructor function name. So:

var what = Object.prototype.toString;
console.log( Date())); // "[object Date]"
console.log({})); // "[object Function]"
console.log([])); // "[object Array]"

...etc. Note that this is very different from just calling toString on the object itself; the object probably has an overridden toString. That's why we explicitly use Object.prototype.toString above. (Remember, these things are just functions, not methods.) Object.prototype.toString works on primitives too, because it coerces its argument into an object before checking. So if you call it with a string primitive, you get "[object String]" (and "[object Number]" for numbers, etc.).

The chief advantage of this is that it doesn't care what window the object came from; referring back to our Date object d from another window, we'll get "[object Date]" regardless.

But sadly for our own constructor functions, all we get is "[object Object]":

var what = Object.prototype.toString;
console.log( MyNiftyThing("foo"))); // "[object Object]"

Ah, well...

At one point I was tempted to wrap all of the above (plus support for my own object hierarchy) up into an uber-function that definitively figured out what the thing was. Then I thought: Enh, maybe I should just use the right tool in each given situation.

Speaking of which, our last tool:

Learn to stop worrying and love the duck

When reaching for instanceof or typeof or whatever, ask yourself: Do you really care? How 'bout looking to see if the object seems to have the things on it you need (feature detection) rather than worrying about what it is? This is usually called "duck typing," from the phrase "If it walks like a duck and talks like a duck..." Sometimes, obviously, you do care, but if you get in the habit of asking yourself the question, it's interesting how frequently the answer is "Actually, no, I don't care whether that's a Foo; I just care that it has X on it" or "...I just care that it works if I pass it into getElementById" (that latter case being, basically, it's a string or its toString does what you need). I find myself doing a lot less worrying about types and just getting on with the job these days. I've learned to love the duck. (And always remember, be kind to your web-footed friends; a duck may be somebody's mother.)

Happy coding!

Postscript: You may be wondering why I haven't mentioned the constructor property in any of the above. The answer is: Because it's not very useful, and there are common anti-patterns that mess it up. At some point I'll do an article on what's wrong with constructor...


Anonymous said...

This is nice. I am using it in one of my Thunderbird Addons, which creates bookmarks menu (a reading list) from currently selected messages. I already had code written for drag & drop but also had a "add current item" menu item which would look at the currently loaded mail (single message tab) or content URL (if a web page was visited in a content Tab); however in the thread pane it would only pick up the first selected message - the function [getActiveUri()] would return a single Object and I rewrote it to return an Array of these just in this special case (multiple messages / conversation selected).

So I needed to distinguish this case and do some special code based on the "Array-ness" of the returned object, without having to rewrite the whole function (which would return a lot of different single objects dependant on context). Another option might have been to

let urlResult = this.getActiveUri();

and then add array evaluation code anyway, but I find that little to obfuscating. I will still have to understand the code in 5 years time.

Martijn.s said...

what about ?