49 lines
1.4 KiB
Markdown
49 lines
1.4 KiB
Markdown
### Classes are compiled to ES2015 classes
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ES2015 classes and their methods have some restrictions beyond those on regular functions.
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Class constructors can’t be invoked without `new`:
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```coffee
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(class)()
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# Throws a TypeError at runtime
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```
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Derived (extended) class `constructor`s cannot use `this` before calling `super`:
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```coffee
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class B extends A
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constructor: -> this # Throws a compiler error
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```
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ES2015 classes don’t allow bound (fat arrow) methods. The CoffeeScript compiler goes through some contortions to preserve support for them, but one thing that can’t be accommodated is calling a bound method before it is bound:
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```coffee
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class Base
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constructor: ->
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@onClick() # This works
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clickHandler = @onClick
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clickHandler() # This throws a runtime error
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class Component extends Base
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onClick: =>
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console.log 'Clicked!', @
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```
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Class methods can’t be used with `new` (uncommon):
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```coffee
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class Namespace
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@Klass = ->
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new Namespace.Klass # Throws a TypeError at runtime
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```
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Due to the hoisting required to compile to ES2015 classes, dynamic keys in class methods can’t use values from the executable class body unless the methods are assigned in prototype style.
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```coffee
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class A
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name = 'method'
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"#{name}": -> # This method will be named 'undefined'
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@::[name] = -> # This will work; assigns to `A.prototype.method`
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```
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