Arrays

Mapping values

You want to convert all elements in an array to some other form.

For example, you have

theUsers = [
  {id: 1, username: 'john'}
  {id: 2, username: 'lexy'}
  {id: 3, username: 'pete'}
]

and you want to have an array of usernames only, i.e.

['john', 'lexy', 'pete']

Method 1 - using .map

theUsernames = theUsers.map (user) -> user.username

Method 2 - using comprehension

theUsernames = (user.username for user in theUsers)

Filtering values

theUsers = [
  {id: 1, username: 'john'}
  {id: 2, username: 'lexy'}
  {id: 3, username: 'pete'}
]

To retain only users whose id is greather than 2, use the following:

[{id: 3, username: 'pete'}]

Method 1 - using .filter

filteredUsers = theUsers.filter (user) -> user.id >= 2

Method 2 - using comprehension

filteredUsers = (user for user in theUsers when user.id >= 2)

Slicing

If you want to extract a subset of an array (i.e. numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]) you can easily do this with one of the following examples:

• numbers[0..2] will return [1, 2, 3]
• numbers[3...-2] will return [3, 4, 5, 6]
• numbers[-2..] will return [8, 9]
• numbers[..] will return [1, 2, 3, 4, 5, 6, 7, 8, 9]

With two dots (3..6), the range is inclusive [3, 4, 5, 6]
With three dots (3…6), the range excludes the end [3, 4, 5]
Adding a - to the range will start the count at the end of the array
An omitted first index defaults to zero
An omitted second index defaults to the size of the array

The same syntax can be used with assignment to replace a segment of an array with new values

numbers[3..6] = [-3, -4, -5, -6]

The above row will replace the numbers array with the following : [1, 2, -3, -4, -5, -6, 7, 8, 9]

Concatenation

You want to combine arrays into one.

For example, you have

fruits = ['Broccoli', 'Carrots']
spices = ['Thyme', 'Cinnamon']

and you want to combine them into

ingredients = ['Broccoli', 'Carrots', 'Thyme', 'Cinnamon']

Method 1 - using .concat

ingredients = fruits.concat spices

Method 2 - using splats

ingredients = [fruits..., spices...] 

Method 3 - using .concat with indeterminate number of arrays

If the number of arrays can vary, e.g. you have array of arrays:

arrayOfArrays = [[1], [2,3], [4]]
[].concat.apply([], arrayOfArrays) # [1, 2, 3, 4]

Comprehensions

You can do neat things via the results of Array “comprehensions”…

Like assign multiple variables… from the result of a looping for statement…

[express,_] = (require x for x in ['express','underscore'])

Or a syntactically sweet version of a “mapped” function call, etc…

console.log (x.nme for x in [{nme:'Chad',rnk:99}, {nme:'Raul', rnk:9}])

[ 'Chad', 'Raul' ]

Notice the ( ) surrounding these statements. These parenthesis are required to make the enclosed comprehension “work”.