After wrestling with another 140bytes challenge, I found myself wondering how many different ways you can test an object in JS to see if it’s a function. I wrote out a few of them then threw it out to my colleagues who came up with a few more. I’d love to hear from anyone who can suggest more to add to the list. Ideally, you want to find a test that will return true for all functions and only for functions. It’d be great if it’s a universal test that can be slightly modified to test for other types but that’s not essential.
Bear in mind, most of these shouldn’t be used in the real world, this is just for fun.
There are a couple of main categories and several variations within.
Duck Typing
When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.
In other words, if an object has the methods of a function and the properties of a function, it’s probably a duck. No, that doesn’t sound right.
This simply detects whether the object we’re testing contains the methods or properties we’d normally associate with a function. The most common test is checking whether a.call exists. In most cases, this will only be defined on a function and it will be defined on all functions. It is, therefore, a good test to use.
The downside is that it can be fooled by setting a property of call on the object to be truthy. This will pass the test but still not be a function. Also, if the object is null or undefined, this will throw a TypeError (as pointed out by subzey on git).
a.call // Hey, this has a call property. Quack?
a.call && a.call.call // The call also has a call so is probably also a function. Quack.
a.apply // As before but with apply this time
a.apply && a.apply.apply // ditto and ditto
String comparisons on typeof
This area of inspection is probably the richest for coming up with silly ideas to test. The typeof operator simply returns a string containing the type of the object. That’s it. Anything you can think of to compare a string against another string, be it RegEx, charAt, equals or threequals (===) can be manipulated to become a check for type.
typeof a>'f'&&typeof a<'g' // Long-winded and quite silly. Performs a numerical comparison on the strings
(typeof a).charAt(0)=='f' // Sturdy but not very useful.
typeof a==typeof eval // May as well use eval for something, it’s a global function
typeof a==typeof dir // Shorter but might not exist everywhere
typeof a=='function' // The usual way to test. Everything above here is purely academic
/^f/.test(typeof a) // Matching the string against a unique RegEx. See the table below
typeof a==typeof b // Requires access to another variable which is a known function
(typeof a)[0]=='f' // Small and robust but doesn’t work in IE6 or 7
Table of RegEx Patterns to match object types:
As little aside here, we’ve got a table of simple RegEx tests that do the same as the one mentioned above. They return true if the type is what you expect, false for all other types. They work by assuming things like ‘object’ being the only type to contain a ‘j’ or ‘boolean’ being the only one with an ‘a’.
Type RegEx Note
boolean /a/.test(typeof a) // Shorter than typeof a==‘boolean’
function /^f/.test(typeof a) // Shorter than typeof a==‘function’
undefined /d/.test(typeof a) // Shorter than typeof a==‘undefined’
number /m/.test(typeof a) // Same length as typeof a==‘number’
object /j/.test(typeof a) // Same length as typeof a==‘object’
string /s/.test(typeof a) // Same length as typeof a==‘string’
null /ll/.test(typeof a) // Longer than typeof a==‘null’
Pick & Mix
This not only makes the assumption that an object is probably a function if it contains a ‘call’ but also that if that call has the same type as the object, they’re both probably functions.
typeof a==typeof a.call // A mixture of typeof string comparison and duck typinginstanceof
In some circumstances, instanceof is going to be better than typeof as it compares types rather than strings.
a instanceof Function // This will throw a ReferenceError if a is undefined.The [[Class]] of the object
This comes from the JavaScript Garden where you’ll find they have a strong opinion on typeof and instanceof. This uses call to execute the toString method on the prototype of the basic Object constructor. Phew. At that point, you'd have a string ‘[Object Function]’. You can then chop off the beginning and the end using slice (treating the string as if it were an array) to get just the type. All together, it looks like this:
Object.prototype.toString.call(a).slice(8, -1);Testing the string representation of the object
This is fairly nasty but still quite effective. Convert the object itself to a string (not its type but the actual object) and see if that begins with ‘function’. This is nowhere nearly as robust as some of the other tests as this will also pass true for any strings that begin "function..." but it’s quite cunning. Credit goes to Max for this one.
/^function/.test(a.toString()) //Test if the output of .toString() begins with ‘function’
/^function/.test(a+"") //As above but using a shortcut to coerce the function to a string.
Running it
This isn’t so much checking whether it looks and sounds like a duck, this is more like serving it à l’orange and seeing if it tastes like a duck. The idea here is to actually try and execute it. If it throws an error, it’s not executable, if it doesn’t, it is. Or something like that. Here, we’re testing that the error is an instanceof TypeError as an undefined object would also end up in the catch.
The obvious downfall to this technique is that you don’t necessarily want the function executed when you’re testing it. In fact, you almost never want to do that. I might go as far as to say you never want that.
try {
a();
true;
} catch (e) {
!(e instanceof TypeError);
}
The other big weakness in the above technique is that, even if the object is a function, the call itself might throw a TypeError. In Chrome, there's a bit more transparency as the error thrown has a type property. In that case you want to check that the type is 'called_non_callable' but that might still be a consequence of the function. In Safari, there's a similar property on the error (e.message) but the error object itself is no longer a TypeError, it is just an Error.
More…
I’m certain there are more. Many, many more. There are also several dozen that are trivial variations on those mentioned above – you could do the same string comparison tests on the [[Class]] property, for instance – but I missed these out. There’s probably a huge section missed out here (I'd forgotten instanceof existed until after the first draft of this post, for instance). If you can think of any more, let me know here or on The Twitter.
I'll also reiterate my point from earlier: most of these are deliberately useless or inefficient. The point here isn't to find better ways to do things, it's to practice doing what you do every day. The more you play while being a web developer, the less you need to work.
