The Closure Compiler is a minifier, an optimiser and a validator all-in-one. That kind of puts it in its own category, because you’re correct that a compiler should at least take something that won’t run in its current form and turn it into something that will (take TypeScript for an ECMAScript-based example).

But do you blame Google for stretching the terminology? What else were they going to call it? Google Minifier? No, it’s more than that, and there are hundreds of those out there. Google Optimiser? It’s way more than that. Google Validator? No, it’s way more than that too.

So the choice is

• Call it Google Closure Foogle and introduce a whole new otherwise-meaningless word into the lexicon.
• Call it Google Closure Minoptivalidator, which is clearer in intent but harder to remember.
• Call it Google Closure Compiler, which is pretty close to the truth.

It does everything you would expect a compiler to do, with only a semantic difference. And, in the end, all words are defined by their usage, to some extent. So if Google can convince people to call this a compiler, the definition of compiler changes slightly. Certainly not in any way that will cause a problem.

Or, to come back to the earlier example, can you find anything significant about TypeScript that allows it to be called a “true compiler”, while Google Closure Compiler should be restricted to “almost a compiler”?

Based on that, I would say that Google Closure is not a compiler.

Well yes … but that assumes that Wikipedia is authoritative on this point.

And as a counter to the Wikipedia definition, consider some dictionary definitions (from http://www.thefreedictionary.com/compiler):

2) (Computer Science) – A program that translates another program written in a high-level language into machine language so that it can be executed.

2) (Electronics & Computer Science / Computer Science) a computer program by which a high-level programming language, such as COBOL or FORTRAN, is converted into machine language that can be acted upon by a computer.

A computer program associated with certain programming languages that converts the instructions written in those languages into machine code that can later be executed directly by a computer.

Granted these definitions are all a bit old-fashioned, but it does illustrate that there is no “one true meaning” … and indeed that meanings of terms like “compiler” change over time. (And, IMO that is just fine, because we don’t really need a precise definition for this case.)

Debating whether the Closure compiler is a “true compiler” or not is (IMO) not a fruitful activity. It would be more useful to understand what the Google folks mean by “true compiler” … in that context.

I think the reason why it can be called “a real compiler” is because it builds a complete AST (abstract semantic tree) out of your programs, and uses it to generate a new text.

The fact that both the original and resultant texts are valid JavaScript is mere coincidence.

This is important because there are many tools that only do text manipulations of the code (minifiers, prettifiers, etc) but doesn’t do any AST handling, much less code re-generation. Even if those tools are more and more powerful, they’re not the same kind of software, and the limitations are different.

It’s semantics in my opinion. In the traditional sense, it is not a compiler. However, in the same Wikipedia link, it says

A compiler is likely to perform many or all of the following
operations: lexical analysis, preprocessing, parsing, semantic
analysis (Syntax-directed translation), code generation, and code
optimization.

Closure performs some or all of those operations.

A little farther down in the Wikipedia article

However, in practice there is rarely anything about a language that
requires it to be exclusively compiled or exclusively interpreted,
although it is possible to design languages that rely on
re-interpretation at run time.

That being said, my guess is that it was easier to call it the “Closure Compiler” rather than the “Closure Optimizer” because really it is just optimizing JavaScript for browsers, not necessary translating it into another language or bytecode.

Closure Compiler as a noun is a non-sequitur.

First, lets get this out of the way source-to-source compiler exists. You cannot say something isn’t a compiler because the end result isn’t a code of lower level.

While it is correct that Closure Compiler generate AST and have code generation phrase, many Javascript minimizer out there already does that and I would probably not call them compilers neither.

What truly sets Closure Compiler apart is it’s application of many well known compiler techniques in error detection and optimization. Here are some examples:

Type system:

Closure Compiler defines an annotated type system. It utilizes type inference techniques that many compiler uses to check your program for errors.

Interprocedural optimizations

Closure Compiler builds call graphs to rename and remove dead code in a whole program level.

Intraprocedural optimizations

Closure Compiler optimizes Javascript by apply different Control Flow Analysis as well as Data Flow analysis. Classic compiler techniques such as: Inlining, Register Allocation, Live Variable Analysis.. and many more are all utilized to squeeze that last byte out of the output Javascript.

Module System

A lesser known part of Closure Compiler that can be used to divide your code into seperate download for faster start-up time. It utilizes many graph algorithms as well. Something is also very common in “traditional” compilers.

All these compiler and static analysis techniques is what sets Closure Compiler apart from other Javascript minimizer. Taking away from the fact that the targeted input and output language is the same, I don’t see why it isn’t classified as a “true compiler”.