In Ruby, the Enumerable module mixes into collection classes and relies on the class serving up an each
method which yields each item in the collection.
Okay, so if I want to use Enumerable in my own class, I'll just implement each
[1]:
class Colors
include Enumerable
def each
yield "red"
yield "green"
yield "blue"
end
end
> c = Colors.new
> c.map { |i| i.reverse }
#=> ["der", "neerg", "eulb"]
That works as expected.
But if I override the each
method on an existing class with Enumerable, it doesn't break functions like map
. Why not?
class Array
def each
puts "Nothing here"
end
end
> [1,2,3].each {|num| puts num }
#=> "Nothing here"
> [1,2,3].map {|num| num**2 }
#=> [1,4,9]
[1]: from http://www.sitepoint.com/guide-ruby-collections-iii-enumerable-enumerator/
The main difference between map and forEach is that the map method returns a new array by applying the callback function on each element of an array, while the forEach method doesn't return anything. You can use the forEach method to mutate the source array, but this isn't really the way it's meant to be used.
The returning value The first difference between map() and forEach() is the returning value. The forEach() method returns undefined and map() returns a new array with the transformed elements. Even if they do the same job, the returning value remains different.
When I modify the tests to actually do the same thing, map is faster than foreach. I would argue it is scientific when used for the case of "I need to do something to each array element, what should I use?" question.
The map() method in JavaScript creates an array by calling a specific function on each element present in the parent array. It is a non-mutating method. Generally map() method is used to iterate over an array and calling function on every element of array.
Enumerable
's implementation of map
does use each
, but there's nothing stopping an extending class from overriding it with its own implementation that does not use each
.
In this case Array
does provide its own implementation of map
for efficiency reasons.
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