CoffeeScript is a little language that compiles into JavaScript. Think of it as JavaScript's less ostentatious kid brother — the same genes, the same accent, 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 walls 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 can pass 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
While Loops
Array Comprehensions
Array Slice Literals
Calling Super from a Subclass
Embedded JavaScript
Aliases
Switch/Case/Else
Try/Catch/Finally
Multiline Strings
CoffeeScript on the left, compiled JavaScript output on the right.
<%= code_for('overview', 'cubed_list') %>sudo 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. 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 named 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. |
-e, --eval |
Compile and print a little snippet of CoffeeScript directly from the
command line. 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. |
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. In all of the following examples, the source CoffeeScript is provided on the left, and the direct compilation into JavaScript is on the right. Familiarity with JavaScript is assumed, although it would certainly be nice to have a tutorial that builds from the ground up in the future.
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 function, if-statement, switch, or try/catch.
Functions and Invocation Let's start with the best part, shall we? 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 defined within lexical scope — you never need to declare 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 new_num, in the last line.
Although suppressed within this documentation, 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 CoffeScript 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 statements, switch statements, and if-elses (although the ternary operator is preferred).
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()') %>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 !.
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') %>Switch/Case/Else Switch statements in JavaScript are rather broken. You can only do string comparisons, 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. To specify the default case, just use else.
<%= code_for('switch') %>Try/Catch/Finally Try/catch statements just about the same as JavaScript (although they work as expressions). No braces required.
<%= code_for('try') %>Multiline Strings Multiline strings are allowed in CoffeeScript.
<%= code_for('strings') %>