Mixed feelings about JavaScript

Let me say it out clear: there is no Mixed type in JavaScript.

Some time ago our code contained a lot of comments that documented function parameters as Mixed. For example:

 * Adds item to container.
 * @param {Mixed} item Any type of item you wish to add.
function add(item) {

Then somebody came and told me: there is no Mixed type in JavaScript. Oh yeah, I realized, a variable can't be of type X when there is no such type. How silly of me.

But how to document such an anything-goes parameter then? Luckily enough, everything in JavaScript is an object, all inheriting from Object. So if I just specify the type as Object, it will essentially mean that the value can be any JavaScript object.

And so I did. Replacing all the occurrences of Mixed with Object. Well... not all. Often the variable wasn't of anything-goes type, rather an alteration between a select few types. Like it could have been either Number or String, but the author was too lazy, so he just wrote Mixed.

Oh what a happy man I was when I went through and corrected all these bogus type definitions. And I made JSDuck throw a warning when it sees Mixed or any other unknown type. All was well...

...and then it came haunting me.

You know, everything really inherits from Object in JavaScript:

> Object.prototype.foo = 123
> "hello".foo
> (5).foo

But everything doesn't quite act like an object:

> x = "hello"
> x.foo = 123
> x.foo
> x instanceof Object
> x instanceof String

Therefore there is quite a bit difference between a function that accepts an Object:

 * Copies properties of second object into first one.
 * @param {Object} obj1
 * @param {Object} obj2
function merge(obj1, obj2) {
    for (var i in obj2) {
        obj1[i] = obj2[2];

and a function that accepts any value:

 * Wraps item inside array if it's not an array already.
 * @param {Object} obj
 * @return {Array}
function toArray(obj) {
    return obj instanceof Array ? obj : [obj];

But you can't tell the difference if you look at the documented parameter types. That's not good. The role of the documentation should be to tell you, which is which.

Let me repeat it again: there is no Mixed type in JavaScript. But this doesn't mean the Mixed type can't have it's place in the documentation.

One should be careful though. Use of Mixed should be the last resort, reserved for cases where you really accept any value whatsoever.

With that being said, I'm reintroducing the support of Mixed type into JSDuck.

Kirjutatud 30. septembril 2011.


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