It is valid JavaScript to write something like this:
function example(x) {
"Here is a short doc what I do.";
// code of the function
}
The string actually does nothing. Is there any reason, why one shouldn't comment his/her functions in JavaScript in this way?
Two points I could think of during wiriting the question:
The string literal must be initiated, which could be costly in the long run
The string literal will not be recognized as removable by JS minifiers
Any other points?
Edit: Why I brought up this topic: I found something like this on John Resig's Blog, where the new ECMA 5 standard uses a not assigned string literal to enable "strict mode". Now it was my interest to just evaluate, if there could be uses or dangers in doing such documentation.
There's really no point in doing this in Javascript. In Python, the string is made available as the __doc__ member of the function, class, or module. So these docstrings are available for introspection, etc.
If you create strings like this in Javascript, you get no benefit over using a comment, plus you get some disadvantages, like the string always being present.
I was looking for a way to add multi-line strings to my code without littering it with \n's. It looks like this module fits the bill:
https://github.com/monolithed/doc
Unfortunately, the comments won't survive minification, but I suppose you could write a compile task to convert docstrings to "\n" format.
Related
There seems to be a lot of resources for XML pull parsing, but is it possible to build a pull parser for JavaScript? Why is this not something people pursue? Pull parsing enables to stream the file while parsing it, which allows for infinitely sized files (for example) and concurrent use.
The problem I encounter is that I need to divide the code into certain small units. I thought statements would be a good way to split the code. Each call to the pull parser would yield another statement (or function declaration). However this goes wrong with function expressions. They require to split the statements up because each statement could contain a function with more statements.
How would I go about implementing such a parser? Or do you think this is an unwise design?
I'm trying to build a fast minifier.
EDIT: see http://www.infoq.com/articles/HIgh-Performance-Parsers-in-Java-V2 for more info on sequential access parsers. They only describe JSON and XML...
Also see https://github.com/qfox/zeparser2 for a fast streaming JS parser.
EDIT2:
I can think of a few options:
return each grammar type, even nested ones. So (most) tokens will be returned multiple times in different grammars (like an expression inside a statement). So for example you first return the statement 'var a = b + c;' and then return the expression 'b + c'. So as caller you can check if the returned grammar is a var-statement and do something with that...
work with event function, this is push-parsing. Like call the var-statement handler, or expression handler.
full blown AST generation with early return?
I'm trying to understand how JS is actually parsed. But my searches either return some ones very vaguely documented project of a "parser/generator" (i don't even know what that means), or how to parse JS using a JS Engine using the magical "parse" method. I don't want to scan through a bunch of code and try all my life to understand (although i can, it would take too long).
i want to know how an arbitrary string of JS code is actually turned into objects, functions, variables etc. I also want to know the procedures, and techniques that turns that string into stuff, gets stored, referenced, executed.
Are there any documentation/references for this?
Parsers probably work in all sorts of ways, but fundamentally they first go through a stage of tokenisation, then give the result to the compiler, which turns it into a program if it can. For example, given:
function foo(a) {
alert(a);
}
the parser will remove any leading whitespace to the first character, the letter "f". It will collect characters until it gets something that doesn't belong, the whitespace, that indicates the end of the token. It starts again with the "f" of "foo" until it gets to the "(", so it now has the tokens "function" and "foo". It knows "(" is a token on its own, so that's 3 tokens. It then gets the "a" followed by ")" which are two more tokens to make 5, and so on.
The only need for whitespace is between tokens that are otherwise ambiguous (e.g. there must be either whitespace or another token between "function" and "foo").
Once tokenisation is complete, it goes to the compiler, which sees "function" as an identifier, and interprets it as the keyword "function". It then gets "foo", an identifier that the language grammar tells it is the function name. Then the "(" indicates an opening grouping operator and hence the start of a formal parameter list, and so on.
Compilers may deal with tokens one at a time, or may grab them in chunks, or do all sorts of weird things to make them run faster.
You can also read How do C/C++ parsers work?, which gives a few more clues. Or just use Google.
While it doesn't correspond closely to the way real JS engines work, you might be interested in reading Douglas Crockford's article on Top Down Operator Precedence, which includes code for a small working lexer and parser written in the Javascript subset it parses. It's very readable and concise code (with good accompanying explanations) which at least gives you an outline of how a real implementation might work.
A more common technique than Crockford's "Top Down Operator Precedence" is recursive descent parsing, which is used in Narcissus, a complete implementation of JS in JS.
maybe esprima will help you to understand how JS parses the grammar. it's online
Does anyone know of a Javascript lexical analyzer or tokenizer (preferably in Python?)
Basically, given an arbitrary Javascript file, I want to grab the tokens.
e.g.
foo = 1
becomes something like:
variable name : "foo"
whitespace
operator : equals
whitespace
integer : 1
http://code.google.com/p/pynarcissus/ has one.
Also I made one but it doesn't support automatic semicolon insertion so it is pretty useless for javascript that you have no control over (as almost all real life javascript programs lack at least one semicolon) :) Here is mine:
http://bitbucket.org/santagada/jaspyon/src/tip/jaspyon/
the grammar is in jsgrammar.txt, it is parsed by the PyPy parsing lib (which you will have to download and extract from the pypy source) and it build a parse tree which I walk on astbuilder.py
But if you don't have licensing problems I would go with pynarcissus. heres a direct link to look at the code (ported from narcissus):
http://code.google.com/p/pynarcissus/source/browse/trunk/jsparser.py
I need to split a JavaScript file into single instructions. For example
a = 2;
foo()
function bar() {
b = 5;
print("spam");
}
has to be separated into three instructions. (assignment, function call and function definition).
Basically I need to instrument the code, injecting code between these instructions to perform checks. Splitting by ";" wouldn't obviously work because you can also end instructions with newlines and maybe I don't want to instrument code inside function and class definitions (I don't know yet). I took a course about grammars with flex/Bison but in this case the semantic action for this rule would be "print all the descendants in the parse tree and put my code at the end" which can't be done with basic Bison I think. How do I do this? I also need to split the code because I need to interface with Python with python-spidermonkey.
Or... is there a library out there already which saves me from reinventing the wheel? It doesn't have to be in Python.
Why not use a JavaScript parser? There are lots, including a Python API for ANTLR and a Python wrapper around SpiderMonkey.
JavaScript is tricky to parse; you need a full JavaScript parser.
The DMS Software Reengineering Toolkit can parse full JavaScript and build a corresponding AST.
AST operators can then be used to walk over the tree to "split it". Even easier, however, is to apply source-to-source transformations that look for one surface syntax (JavaScript) pattern, and replace it by another. You can use such transformations to insert the instrumentation into the code, rather than splitting the code to make holds in which to do the insertions. After the transformations are complete, DMS can regenerate valid JavaScript code (complete with the orignal comments if unaffected).
Why not use an existing JavaScript interpreter like Rhino (Java) or python-spidermonkey (not sure whether this one is still alive)? It will parse the JS and then you can examine the resulting parse tree. I'm not sure how easy it will be to recreate the original code but that mostly depends on how readable the instrumented code must be. If no one ever looks at it, just generate a really compact form.
pyjamas might also be of interest; this is a Python to JavaScript transpiler.
[EDIT] While this doesn't solve your problem at first glance, you might use it for a different approach: Instead of instrumenting JavaScript, write your code in Python instead (which can be easily instrumented; all the tools are already there) and then convert the result to JavaScript.
Lastly, if you want to solve your problem in Python but can't find a parser: Use a Java engine to add comments to the code which you can then search for in Python to instrument the code.
Why not try a javascript beautifier?
For example http://jsbeautifier.org/
Or see Command line JavaScript code beautifier that works on Windows and Linux
Forget my parser. https://bitbucket.org/mvantellingen/pyjsparser is great and complete parser. I've fixed a couple of it's bugs here: https://bitbucket.org/nullie/pyjsparser
I have a web application that uses TONS of javascript, and as usual, there are a lot of textual constants that will be displayed to the user embedded in the code itself.
What do you think is the best way to make this localizable?
I know I need to take those strings off of the code and replace them with constants, which will be defined into some external place.
For the server side, ASP.Net provides some very neat capabilities for dealing with this.
What's the best to do this in Javascript?
The best idea I have is to have a JS file with ALL the string constants of the JS of the site (i'd have different copies of this, for each language), and then on each page, I include this script first, before all the others.
This seems like the most centralized way, that also wastes the least bandwidth.
Are there any other better approaches?
Thanks!
here's how we did it (in ASP.net), pretty much along the lines of what you've mentioned:
1) Created two javascript files: one which defines all javascript functions/DOM manipulations as required by the site, and, second called Messages.js: this defines all the string literals that need to be localized, something like var ALERT_MSG = "Alert message in english".
2) Created different version of the Messages.js, one for each locale that we are supporting and localized the strings. The localized js files were named using messages.locale.js naming convention (for eg. messages.fr-FR.js).
3) Included the js files within the "ScriptManager" and provided the ResourceUICultures for the Messages.js file: this ensures that the correct localized file is embedded in the html output (if you are not using ASP.net you can build this basic functionality by doing some culture sniffing and including the appropriate js file).
4) Voila!
Your approach makes sense. Details:
I'd have the strings for each language in an object.
localized={"cat":"chat","dog":"chien"};
Then in code:
localized["cat"]
The quotations around of the keys and the array notation (rather than the more common object dot notation) are to avoid collisions with JavaScript reserved words.
There is a gettext library but I haven't used it.
Your approach sounds good enough.
If you have lots of strings and you are concerned about the bulkiness of the file you may want to consider a script that creates a single javascript file for each language by concatenating the code javascript and the locale javascript and then applying something like Minify.
You'll waste some CPU cycles on publishing but you'll save some round trips...
There's a library for localizing JavaScript applications: https://github.com/wikimedia/jquery.i18n
The strings are stored in JSON files, as pretty much everybody else suggests, but it has a few more features:
It can do parameter replacement, supports gender (clever he/she handling), number (clever plural handling, including languages that have more than one plural form), and custom grammar rules that some languages need.
The only requirement is jQuery.