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* Docs: named functions and function declarations * No more prototypal `extends`; update docs and example * More comprehensive documentation of the existential operator; closes #1631 * Better document operators, including `from` * No fat arrow class methods anymore * Destructuring shouldn’t say that default values are applied in case of undefined or null * Spinoff generator and async functions into their own sections; reorder things so that the sections on functions come just before classes, and destructuring goes next to the operators (which discuss assignment) * Rewrite “CoffeeScript 2” section, making it less practical and more explanatory; move practical info into “Usage” * Update “Variable Scoping and Lexical Safety” section to remove incorrect reference to Ruby (fixes #2360), add missing details about the safety wrapper, add note about `let`/`const`. * Updated browser compiler * Updated docs * Rewrite Literate CoffeeScript breaking changes * Split apart the “Breaking Changes” and “Unsupported Features” sections into separate sidebar items and files * Add example of `not in`, closes #3281 * Fix words in bold that should be in backticks * Consolidate some breaking changes sections * Add Node API documentation; closes #3551 * Move the chaining documentation out of the changelog into its own section
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The Existential Operator
It’s a little difficult to check for the existence of a variable in JavaScript. if (variable) … comes close, but fails for zero, the empty string, and false (to name just the most common cases). CoffeeScript’s existential operator ? returns true unless a variable is null or undefined or undeclared, which makes it analogous to Ruby’s nil?.
It can also be used for safer conditional assignment than the JavaScript pattern a = a || value provides, for cases where you may be handling numbers or strings.
codeFor('existence', 'footprints')
Note that if the compiler knows that a is in scope and therefore declared, a? compiles to a != null, not a !== null. The != makes a loose comparison to null, which does double duty also comparing against undefined. The reverse also holds for not a? or unless a?.
codeFor('existence_declared')
If a variable might be undeclared, the compiler does a thorough check. This is what JavaScript coders should be typing when they want to check if a mystery variable exists.
codeFor('existence_undeclared')
The accessor variant of the existential operator ?. can be used to soak up null references in a chain of properties. Use it instead of the dot accessor . in cases where the base value may be null or undefined. If all of the properties exist then you’ll get the expected result, if the chain is broken, undefined is returned instead of the TypeError that would be raised otherwise.
codeFor('soaks')
For completeness:
| Example | Definition |
|---|---|
a? |
tests that a is in scope and a != null |
a ? b |
returns a if a is in scope and a != null; otherwise, b |
a?.b or a?['b'] |
returns a.b if a is in scope and a != null; otherwise, undefined |
a?(b, c) or a? b, c |
returns the result of calling a (with arguments b and c) if a is in scope and callable; otherwise, undefined |
a ?= b |
assigns the value of b to a if a is not in scope or if a == null; produces the new value of a |