incompatibilities

behavior

• Assigning to a variable with = declares it on the current scope. Use := to modify variables declared on upper scopes.
• ===/!==/==/!= each compiles as is.
• super represents the direct reference to the parent function rather than being a call. Use super ... (just super in Coffee) for full delegation.
• Nested comprehensions return flattened results.
• Within non-triple-quoted strings (''/""), newlines and indentations that follow them are ignored.

keyword

• The roles of in and of have been swapped to keep the JS semantics.
• yes/no/on/off are unreserved. Define your own or just use true/false.
• switch-case-default replaces switch-when-else.
• for ever replaces loop.
• is not replaces isnt.
• void replaces undefined.
• ~> replaces =>.
• when is removed. Use a if b while c instead of a while c when b.
• it/that/fallthrough are semi-reserved. They get special meanings depending on context.
• eval/arguments are keywords as per ES5.

grammar

• () is always a call. f () -> means f()(->) (compile error in Coffee).
• ... is prefix as per harmony:spread.
• and/or close implicit calls. f a or f b means f(a) || f(b).
• The range syntax (x..y) is removed.
• ?[ is removed in favor of the generalized form ?.[ (or ?.().
• Block comment is /* */ rather than ### ###.

other

• CLIs are named coco and coke (to coexist with coffee and cake).
• JavaScriptLint support is removed (for being pointless).