Equality Operator

Matrix-agent-Smith-clones One thing that always bothered me, and many others, is how useless the == operator is in JavaScript. It seems to combine the worst of both worlds from equality and identity:

  • When comparing primitive types, it acts as a lax equality operator
  • When comparing objects, it acts as an identity operator

I can’t think of any use case for which such behavior is desired. In fact, reversing this behavior would produce a much more useful operator since the concept of “equals” tends to relax as data gets more complex because some dimensions are simply irrelevant. Do I care that two objects were not created at the same exact time? Do I care that their IDs differ? Do I care if the measured length is 4.00000000001 inches rather than 4? When it comes to equality comparison, JavaScript fails miserably. This is actually one of Douglas Crockford’s pet peeves about the language, which he explains in more detail in his book: JavaScript: The Good Parts.

Fortunately, there is also a more consistent identity operator (===), often erroneously called “strict equality” by people who don’t understand the difference between identity and equality. Unfortunately, as I just mentioned in previous paragraph, there is no concept of equality operator in the language at all. For those unfamiliar with these concepts, identity answers the question of “Does A and B refer to the same object?” while equality answers the question of “Is B a clone of A?“.

Naturally, both of these questions come up in programming a lot, so it’s a shame that only one can be easily answered in JavaScript. You probably already stumbled into this problem if you ever tried something like this:

new Date('1/1/2000') == new Date('1/1/2000')

or this:

{"foo": 1} == {"foo": 1}

The problem is not unique to JavaScript, many other languages lack equality operator as well. Typically, however, those languages have an alternative mechanism for handling this case, such as operator overloading. Indeed, there are cases when operator overloading is actually superior – such as when objects contain meta data that one wishes to omit from comparison (i.e. exact creation time, unique id, etc.). Unfortunately, JavaScript doesn’t allow for operator overloading either, and herein lies the problem. While one can easily roll a proper equality function (and many libraries such as underscore already include one) you would still have to decide whether it’s worth using on a case-by-case basis.

We’re not in FORTRAN age anymore, where developers had to tweak each operation. Today we’re spoiled, we often can get away with simply telling the compiler what to do rather than how, and enjoy optimal performance 90% of the time with negligible overhead. One such example is the sort function. When was the last time you had to wonder if you should use merge or insertion sort? You simply use the built-in sort and assume that unless there is something very special about your data, you’re better off moving on to other things rather than attempting to optimize the last 10% out of the algorithm. In most modern languages equality operator falls in the same category. Sure, you’d be able to shave off a few microseconds by replacing that == (equality) with is (identity) for certain comparison operations in Python, but is it worth the extra brain cycles? Most of the time the answer is “No”.

Why then do we have to be explicitly aware of this in JavaScript? Moreover, why can’t RapydScript compile == into deep equality? First, I should mention that, like some other libraries/languages for JavaScript, RapydScript already has a deep equality comparison via the inbuilt eq function. Yes, eq(a, b) (previously called deep_eq) has been supported for years. The problem is that (up until recently) I wasn’t able to make the decision for you of whether you want eq or ==. The issue boils down to the fact that if I decide to compile == to equality across the board for the developer, I effectively introduce enormous overhead (about 700% according to jsperf) on the user in cases where he/she expected identity comparison instead (which is about 90% of the time, since primitives are a lot more common). There has been a lot of discussion about this, which you can follow in issues 93 and 94 on my github page.

As you probably already guessed from previous paragraph, I’m now able to bridge that gap. Effective last month (that’s right, I snuck a change in without anyone noticing), RapydScript is the first JavaScript-based transcompiler to support high-performance deep equality. How performant is this operation, you may ask? Well, take a look for yourself:

Screen Shot 2015-12-05 at 2.59.38 PM

According to jsperf, the overhead is negligible (that’s right, the measurement noise is greater than any visible difference – as you can see from the deep-equality version outperforming identity). So what changed? Why am I suddenly able to blow the doors off of typical deep equality? Well, nothing new happened in JavaScript world. What changed is my approach. I decided to think about the problem creatively and had a sudden eureka moment (which I’m sure other compilers will copy in the future). Unfortunately you’ve probably already looked at the code from JsPerf above, ruining my surprise. But in case you haven’t, here is the pattern I came up with:

A === B || typeof A === "object" && eq(A, B)

How does it work? As you probably learned in your introductory programming class (assuming it was a legitimate C/Java class rather than an online tutorial about JavaScript), binary operators have short-circuit ability in just about all languages. This means that if left-hand side is truthy for || (or) operator, or falsy for && (and), the rest of the line is ignored (it’s as if it’s not there). That means that if A and B are primitives that are equal, the above will be equivalent to a simple A === B comparison. That handles the positive case, but negative is a bit trickier. I was struggling with it for a while (yes, the above equation makes it seem simpler than it really is – everything seems easy in hindsight), until I found a performant operation that works in both, browser and node (my first attempt was to check if A.constructor exists, which doesn’t work on all platforms). Fortunately, the very same “feature” that makes typeof useless in most cases (the fact that every non-primitive is an object) becomes the saving grace of this operation. As you can see from the JsPerf test, the overhead for this operation is negligible as well.

The best part is that unlike native JavaScript, where the developer would have to type that out by hand (because hiding it in a function call introduces the 700% overhead we’re trying to avoid), RapydScript can unroll == operator into that magic automatically. You’re probably wondering, then, how is it that this change has been in RapydScript for over a month if the == still compiles to ===? Well, for safety I’ve hidden this operator behind an import. If you wish all your == operators to compile to proper equality test shown above, add the following line at the top of your file:

from danger_zone import equality

That’s it. Now all your == will compile to the magic shown above, and != will compile to its inverse (thanks to DeMorgan’s Law). The above import will also tweak implementation of indexOf and lastIndexOf to perform deep equality as well, which in turn makes tests like if a in arr be based on deep equality (delivering a consistent experience across the board). As before, the identity operator is still there as well, via is and is not, which compile to === and !==, respectively. In the future, I also want to add an optimizer to the compiler, which will strip away the remainder of the line at compile time if one of the operands is a constant.