Some languages, like Ruby, have conventions that make it easy to
tell if a method modifies something in place or returns a copy. For
example, in Ruby, if you have a string str, and you call str.down
case
, it will return a lowercased version, but str will remain
unchanged. On the other hand, if you call str.downcase!, it will
modify str in place. The fact that the standard libraries in Java‐
Script offer no clue about which methods return a copy and which
modify in place is, in my opinion, one of the language’s shortcom‐
ings, requiring unnecessary memorization.
Adding or Removing Single Elements at the Beginning or End
When we refer to the beginning (or front) of an array, we’re referring to the first ele‐
ment (element 0). Likewise, the end (or back) of an array is the largest element (if our
array is
arr
, element
arr.length-1
).
push
and
pop
add and remove, respectively, ele‐
ments to the end of the array (in place).
shift
and
unshift
remove and add, respec‐
tively, elements to the beginning of the array (in place).
The names of these methods come from computer science termi‐
nology. push and pop are actions on a stack, where the important
elements were the most recent ones added. shift and unshift
treat the array like a queue, where the important elements are the
oldest ones added.
push
and
unshift
return the new length of the array after the new element has been
added, and
pop
and
shift
return the element that was removed. Here are examples of
these methods in action:
const
arr
=
[
"b"
,
"c"
,
"d"
];
arr
.
push
(
"e"
);
// returns 4; arr is now ["b", "c", "d", "e"]
arr
.
pop
();
// returns "e"; arr is now ["b", "c", "d"]
arr
.
unshift
(
"a"
);
// returns 4; arr is now ["a", "b", "c", "d"]
arr
.
shift
();
// returns "a"; arr is now ["b", "c", "d"]
Adding Multiple Elements at the End
The
concat
method adds multiple elements to the array and returns a copy. If you
pass
concat
arrays, it will break apart those arrays and add their elements to the orig‐
inal array. Examples:
const
arr
=
[
1
,
2
,
3
];
arr
.
concat
(
4
,
5
,
6
);
// returns [1, 2, 3, 4, 5, 6]; arr unmodified
arr
.
concat
([
4
,
5
,
6
]);
// returns [1, 2, 3, 4, 5, 6]; arr unmodified
arr
.
concat
([
4
,
5
],
6
);
// returns [1, 2, 3, 4, 5, 6]; arr unmodified
arr
.
concat
([
4
, [
5
,
6
]]);
// returns [1, 2, 3, 4, [5, 6]]; arr unmodified
Array Content Manipulation | 133
