CoffeeScript is a little language that compiles into JavaScript. Think of it as JavaScript's less ostentatious kid brother — the same genes, roughly the same height, but a different sense of style. Apart from a handful of bonus goodies, statements in CoffeeScript correspond one-to-one with their equivalent in JavaScript, it's just another way of saying it.
Disclaimer: CoffeeScript is just for fun and seriously alpha. I'm sure that there are still plenty of holes in the lexer and leaks in the syntax. There is no guarantee, explicit or implied, of its suitability for any purpose. That said, it compiles into clean JavaScript (the good parts) that can use existing JavaScript libraries seamlessly, and passes through JSLint without warnings. The compiled output is quite readable — pretty-printed, with comments preserved intact.
Mini Overview
Installation and Usage
Punctuation Primer
Functions and Invocation
Assignment
Objects and Arrays
Lexical Scoping and Variable Safety
Conditionals, Ternaries, and Conditional Assignment
Everything is an Expression
Aliases
While Loops
Array Comprehensions
Array Slice Literals
Calling Super from a Subclass
Embedded JavaScript
Switch/When/Else
Try/Catch/Finally
Multiline Strings
Change Log
CoffeeScript on the left, compiled JavaScript output on the right.
<%= code_for('overview', 'cubed_list') %>The CoffeeScript compiler is written in pure Ruby, and is available as a Ruby Gem.
gem install coffee-script
Installing the gem provides the coffee-script command, which can be used to compile CoffeeScript .cs files into JavaScript, as well as debug them. By default, coffee-script writes out the JavaScript as .js files in the same directory, but output can be customized with the following options:
-o, --output [DIR] |
Write out all compiled JavaScript files into the specified directory. |
-w, --watch |
Watch the modification times of the coffee-scripts, recompiling as soon as a change occurs. |
-p, --print |
Instead of writing out the JavaScript as a file, print it directly to stdout. |
-l, --lint |
If the jsl (JavaScript Lint) command is installed, use it to check the compilation of a CoffeeScript file. (Handy in conjunction with --watch) |
-e, --eval |
Compile and print a little snippet of CoffeeScript directly from the
command line (or from stdin). For example: coffee-script -e "square: x => x * x." |
-t, --tokens |
Instead of parsing the CoffeeScript, just lex it, and print out the token stream: [:IDENTIFIER, "square"], [":", ":"], [:PARAM, "x"] ... |
-v, --verbose |
As the JavaScript is being generated, print out every step of code generation, including lexical scope and the node in the AST. |
--install-bundle |
Install the TextMate bundle for CoffeeScript syntax highlighting. |
Examples:
coffee-script path/to/script.cs coffee-script --watch --lint experimental.cs coffee-script --print app/scripts/*.cs > concatenation.js
This document is structured so that it can be read from top to bottom, if you like. Later sections use ideas and syntax previously introduced. Familiarity with JavaScript is assumed. In all of the following examples, the source CoffeeScript is provided on the left, and the direct compilation into JavaScript is on the right.
Punctuation Primer You don't need to use semicolons ; to terminate expressions, ending the line will do just as well. All other whitespace is not significant. Instead of using curly braces { } to delimit a block of code, use a period . to mark the end of a block, for functions, if-statements, switch, and try/catch.
Functions and Invocation Functions are defined by a list of parameters, an arrow, and the function body. The empty function looks like this: =>.
<%= code_for('functions', 'cube(5)') %>Assignment Use a colon : to assign, as in JSON. Equal signs are only needed for mathy things.
<%= code_for('assignment', 'greeting') %>Objects and Arrays Object and Array literals look very similar to their JavaScript cousins. When you spread out each assignment on a separate line, the commas are optional. In this way, assigning object properties looks the same as assigning local variables.
<%= code_for('objects_and_arrays', 'song.join(",")') %>Lexical Scoping and Variable Safety The CoffeeScript compiler takes care to make sure that all of your variables are properly declared within lexical scope — you never need to write var yourself.
<%= code_for('scope', 'new_num') %>Notice how the variables are declared with var the first time they appear. The second reference of num, within the function, is not redeclared because num is still in scope. As opposed to the second occurrence of new_num, in the last line.
Although suppressed within this documentation for clarity, all CoffeeScript output is wrapped in an anonymous function: (function(){ ... })(); This safety wrapper, combined with the automatic generation of the var keyword, make it exceedingly difficult to pollute the global namespace by accident.
Conditionals, Ternaries, and Conditional Assignment If/else statements can be written without the use of parenthesis and curly brackets. As with functions and other block expressions, conditionals are closed with periods. No period is necessary when using the single-line postfix form, with the if at the end.
<%= code_for('conditionals') %>CoffeeScript includes the conditional assignment operators: ||:, which only assigns a value to a variable if the variable's current value is falsy, and &&:, which only replaces the value of truthy variables.
Everything is an Expression (at least, as much as possible) You might have noticed how even though we don't add return statements to CoffeeScript functions, they nonetheless return their final value. The CoffeeScript compiler tries to make sure that all statements in the language can be used as expressions. Watch how the return gets pushed down into each possible branch of execution, in the function below.
<%= code_for('expressions', 'eldest') %>The same mechanism is used to push down assignment through switch statements, and if-elses (although the ternary operator is preferred).
Aliases Because the == operator frequently causes undesirable coercion, is intransitive, and has a different meaning than in other languages, CoffeeScript compiles == into ===, and != into !==. In addition, is compiles into ===, and aint into !==.
You can use not as an alias for !.
Instead of a newline or semicolon, then can be used to separate conditions from expressions, in while, if/else, and switch/when statements.
As in YAML, on and yes are the same as boolean true, while off and no are boolean false.
For single-line statements, unless can be used as the inverse of if.
<%= code_for('aliases') %>While Loops The only low-level loop that CoffeeScript provides is the while loop.
<%= code_for('while') %>Other JavaScript loops, such as for loops and do-while loops can be mimicked by variations on while, but the hope is that you won't need to do that with CoffeeScript, either because you're using each (forEach) style iterators, or...
Array Comprehensions For your looping needs, CoffeeScript provides array comprehensions similar to Python's. They replace (and compile into) for loops, with optional guard clauses and the value of the current array index. Unlike for loops, array comprehensions are expressions, and can be returned and assigned. They should be able to handle most places where you otherwise would use a loop, each/forEach, map, or select/filter.
<%= code_for('array_comprehensions') %>Array Slice Literals CoffeeScript includes syntax for extracting slices of arrays. The first argument is the index of the first element in the slice, and the second is the index of the last one.
<%= code_for('slices', 'three_to_six') %>Calling Super from a Subclass JavaScript's prototypal inheritance has always been a bit of a brain-bender, with a whole family tree of libraries that provide a cleaner syntax for classical inheritance on top of JavaScript's prototypes: Base2, Prototype.js, JS.Class, etc. The libraries provide syntactic sugar, but the built-in inheritance would be completely usable if it weren't for one small exception: it's very awkward to call super, the prototype object's implementation of the current function. CoffeeScript converts super() calls into calls against the immediate ancestor's method of the same name.
<%= code_for('super', true) %>Embedded JavaScript If you ever need to interpolate literal JavaScript snippets, you can use backticks to pass JavaScript straight through.
<%= code_for('embedded', 'hi()') %>Switch/When/Else Switch statements in JavaScript are rather broken. You can only do comparisons based on string equality, and need to remember to break at the end of every case statement to avoid accidentally falling through to the default case. CoffeeScript compiles switch statements into JavaScript if-else chains, allowing you to compare any object (via ===), preventing fall-through, and resulting in a returnable, assignable expression. The format is: switch condition, when clauses, else the default case.
<%= code_for('switch') %>Try/Catch/Finally Try/catch statements are just about the same as JavaScript (although they work as expressions).
<%= code_for('try') %>Multiline Strings Multiline strings are allowed in CoffeeScript.
<%= code_for('strings', 'moby_dick') %>0.1.0 Initial CoffeeScript release.