Comparison-to-CoffeeScript.md

Comparison to CoffeeScript

If you are coming to Civet from a CoffeeScript background this summary will be helpful.

Things Kept from CoffeeScript

• is → ===
• or, or= → ||, ||=
• and, and= → &&, &&=
• not → !
• a not instanceof b → !(a instanceof b)
• a %% b → (a % b + b) % b
• loop → while(true)
• unless exp → if(!exp)
• until condition → while(!condition)
• Object literal syntax

  x =
    a: 1
    b: 2
    c:
      x: "pretty"
      y: "cool"

• Optional semi-colons
• Indentation based block syntax
• OptionalChain shorthand for index and function application: a?[b] → a?.[b], a?(b) → a?.(b)
• ?= null-coalescing assignment shorthand
• @ this shorthand: @ → this, @id → this.id, {@id} → {id: this.id}
• Class static shorthand @
• Chained comparisons: a < b < c → a < b && b < c
• Postfix if/unless/while/until/for
• Block Strings """ / '''
  • #{exp} interpolation in """ strings
• when inside switch automatically breaks
• Multiple , separated case/when expressions
• else → default in switch
• Range literals [0...10], [a..b], [x - 2 .. x + 2]
• Array slices list[0...2] → list.slice(0, 2)
• Slice assignment numbers[3..6] = [-3, -4, -5, -6] → numbers.splice(3, 4, ...[-3, -4, -5, -6])
• Implicit returns
• Late assignment x + y = z → x + (y = z)
• Braceless inline objects x = coolStory: true
• Simplified number method calls 1.toFixed() → 1..toFixed()
• if/switch/for/loop/while/throw expressions
• Destructuring object assignment doesn't require being wrapped in parens at the statement level {a, b} = c → ({a, b} = c)
• Prefix or postfix rest/splats [...a], x = [a...]
• RestProperty in any position {a, ...b, c} = d → {a, c, ...b} = d
• RestElement/RestParameter in any position (first, ...midle, last) -> → function(first, ...middle) { let [last] = middle.splice(-1)}
• /// heregexp (with some changes)
• ### block comments (but allowing nested */)
• JSX with improved shorthands and an optional nested syntax (with some changes)

Things Removed from CoffeeScript

Most of these features are not available by default, but can be enabled by adding a "civet coffeeCompat" directive prologue to the top of your file. The goal is to provide a very high level of compatibility with existing CoffeeScript code while offering a fine grained migration path to modern Civet.

• Implicit var declarations (use "civet coffeeCompat" or "civet autoVar")
• on/yes/off/no (use true/false, "civet coffeeCompat", or "civet coffeeBooleans" to add them back)
• do keyword (replaced with JS do...while blocks and new do blocks; replace with ((x) -> ...)(x) syntax or use "civet coffeeCompat", or "civet coffeeDo")
• for from (use JS for of, "civet coffeeCompat", or "civet coffeeForLoops")
• for in (use for each of, or "civet coffeeCompat", or "civet coffeeForLoops")
• for own of (use for own in, or "civet coffeeCompat", or "civet coffeeForLoops")
• for ... when <condition> (use continue if exp inside loop, "civet coffeeCompat", or "civet coffeeForLoops")
• a ? b (use a ?? b, though it doesn't check for undeclared variables; "civet coffeeCompat", or "civet coffeeBinaryExistential" enables a ? b at the cost of losing JS ternary operator)
• a of b (use a in b as in JS, or "civet coffeeCompat", or "civet coffeeOf")
• a not of b (use a not in b, or "civet coffeeCompat", or "civet coffeeOf")
• Prototype shorthand: X:: → X.prototype, X::a → X.prototype.a (use "civet coffeeCompat", or civet "coffeePrototype"). Note that Civet doesn't allow space between the identifier and the ::.
• Backtick embedded JS (replaced by template literals)
• # one-line comments, to make room for private class fields (use //, or "civet coffeeCompat" or "civet coffeeComment" to enable)
• Will add later
  • Conditional assignment a?[x] = 3 → a ? a[x] = 3 : undefined
  • Multiple slice assignment otherNumbers[0...] = numbers[3..6] = [-3, -4, -5, -6]

Things Changed from CoffeeScript

• == → == rather than === (unless you specify "civet coffeeCompat" or "civet coffeeEq")
• != → != rather than !== (unless you specify "civet coffeeCompat" or "civet coffeeEq")
• is not → !== (unless you specify "civet coffeeCompat" or "civet coffeeNot"), instead of isnt (unless you specify "civet coffeeCompat" or "civet coffeeIsnt")
• for in and for of are no longer swapped and become their JS equivalents (unless you specify "civet coffeeCompat" or "civet CoffeeOf")
• a is in b → b.indexOf(a) >= 0 and a is not in b → b.indexOf(a) < 0 instead of a in b and a not in b; a in b remains a in b as in JS, and a not in b → !(a in b) (unless you specify "civet coffeeCompat" or "civet coffeeOf")
• x?.y now compiles to x?.y rather than the if typeof x !== 'undefined' && x !== null if check
• Existential x? → (x != null) no longer checks for undeclared variables.
• x?() → x?.() instead of if (typeof x === 'function') { x() }
• Functions don't implicitly return the last value if there's a semicolon at the end: -> x returns x but -> x; does not
• Backtick embedded JS has been replaced with JS template literals.
• No longer allowing multiple postfix if/unless on the same line (use && or and to combine conditions).
• #{} interpolation in "" strings only when "civet coffeeCompat" or "civet coffeeInterpolation"
• Expanded chained comparisons to work on more operators a in b instanceof C → a in b && b instanceof C
• Postfix iteration/conditionals always wrap the statement #5431: try x() if y → if (y) try x()
• Civet tries to keep the transpiled output verbatim as much as possible. In Coffee (x) → x; but in Civet (x) → (x). Spacing and comments are also preserved as much as possible.
• Heregex / re.X
  • Stay closer to the Python spec
  • Allows both kinds of substitutions #{..}, ${..}.
  • Also allows both kinds of single line comments //, #.
  • Keeps non-newline whitespace inside of character classes.
  • Doesn't require escaping # after space inside of character classes.
  • # is always the start of a comment outside of character classes regardless of leading space (CoffeeScript treats \s+# as comment starts inside and outside of character classes).
  • Might later add a compat flag to get more CoffeeScript compatibility.
  • Might also later add a compat flag to only use ES interpolations and comments inside Heregexes.
• JSX children need to be properly indented (or use "civet coffeeCompat" or "civet coffeeJSX", but then all JSX tags must be explicitly closed)

Things Added that CoffeeScript Didn't

• CoffeeScript improvements
  • Postfix loop run() loop → while(true) run()
  • Character range literals ["a".."z"], ['f'..'a'], ['0'..'9']
• JS Compatibility
  • var, let, const
  • JS Comment Syntax // and /* */
  • function keyword
  • Braced Blocks (as an alternative to indentation)
  • f?.(x) function application and a?.[x] index OptionalChain longhand
  • a ? b : c ConditionalExpression
  • case statement
  • do, do { ... } until condition
  • Method definitions foo(args) ... in objects/classes
  • get/set method definitions
  • Private identifiers #id
• 99% TS Compatibility
  • Type annotations
  • namespace
  • interface
  • TypeParameters
  • ! non-null assertions
  • and all the other TypeScript features you know and love.

CoffeeScript Compatibility

Civet provides a compatibility prologue directive that aims to be 97+% compatible with existing CoffeeScript2 code (still a work in progress).

Configuration	What it enables
autoVar	declare implicit vars based on assignment to undeclared identifiers
coffeeBinaryExistential	x ? y → x ?? y
coffeeBooleans	yes, no, on, off
coffeeClasses	CoffeeScript-style class methods via -> functions
coffeeComment	# single line comments
coffeeDo	do ->, disables ES6 do/while
coffeeEq	== → ===, != → !==
coffeeForLoops	for in, of, from loops behave like they do in CoffeeScript
coffeeInterpolation	"a string with #{myVar}"
coffeeIsnt	isnt → !==
coffeeLineContinuation	\ at end of line continues to next line
coffeeNot	not → !, disabling Civet extensions like is not
coffeeOf	a of b → a in b, a not of b → !(a in b), a in b → b.indexOf(a) >= 0, a not in b → b.indexOf(a) < 0
coffeePrototype	x:: -> x.prototype, x::y -> x.prototype.y

You can use these with "civet coffeeCompat" to opt in to all or use them bit by bit with "civet coffeeComment coffeeEq coffeeInterpolation". Another possibility is to slowly remove them to provide a way to migrate files a little at a time "civet coffeeCompat -coffeeBooleans -coffeeComment -coffeeEq". Both camel case and hyphens work when specifying options "civet coffee-compat". More options will be added over time until 97+% compatibility is achieved.