Concatenating Arrays

Problem

You want to join two arrays together.

Solution

There are two standard options for concatenating arrays in JavaScript.

The first is to use JavaScript’s Array concat() method:

array1 = [1, 2, 3]
array2 = [4, 5, 6]
array3 = array1.concat array2
# => [1, 2, 3, 4, 5, 6]

Note that array1 is not modified by the operation. The concatenated array is returned as a new object.

If you want to merge two arrays without creating a new object, you can use the following technique:

array1 = [1, 2, 3]
array2 = [4, 5, 6]
Array::push.apply array1, array2
array1
# => [1, 2, 3, 4, 5, 6]

In the example above, the Array.prototype.push.apply(a, b) approach modifies array1 in place without creating a new array object.

We can simplify the pattern above using CoffeeScript by creating a new merge() method for Arrays.

Array::merge = (other) -> Array::push.apply @, other

array1 = [1, 2, 3]
array2 = [4, 5, 6]
array1.merge array2
array1
# => [1, 2, 3, 4, 5, 6]

Alternatively, we can pass a CoffeeScript splat (array2...) directly into push(), avoiding the Array prototype.

array1 = [1, 2, 3]
array2 = [4, 5, 6]
array1.push array2...
array1
# => [1, 2, 3, 4, 5, 6]

A more idiomatic approach is to use the splat operator (...) directly in an array literal. This can be used to concatenate any number of arrays.

array1 = [1, 2, 3]
array2 = [4, 5, 6]
array3 = [array1..., array2...]
array3
# => [1, 2, 3, 4, 5, 6]

Discussion

CoffeeScript lacks a special syntax for joining arrays, but concat() and push() are standard JavaScript methods.