JavaScript – Adding and Deleting Array Elements

We’ve already seen the simplest way to add elements to an array:
just assign values to new indexes:

a = []           // Start with an empty array.
a[0] = "zero";   // And add elements to it.
a[1] = "one";

You can also use the push()
method to add one or more values to the end of an array:

a = [];              // Start with an empty array
a.push("zero")       // Add a value at the end.  a = ["zero"]
a.push("one", "two") // Add two more values.  a = ["zero", "one", "two"]

Pushing a value onto an array a is the same as assigning the value to
a[a.length]. You can use the
unshift() method (described in
Array Methods) to insert a value at the beginning of
an array, shifting the existing array elements to higher
indexes.

You can delete array elements with the delete operator, just as you can delete
object properties:

a = [1,2,3];   
delete a[1];   // a now has no element at index 1
1 in a         // => false: no array index 1 is defined 
a.length       // => 3: delete does not affect array length

Deleting an array element is similar to (but subtly different
than) assigning undefined to that
element. Note that using delete on
an array element does not alter the length property and does not shift elements
with higher indexes down to fill in the gap that is left by the
deleted property. If you delete an element from an array, the array
becomes sparse.

As we saw above, you can also delete elements from the end of an
array simply by setting the length
property to the new desired length. Arrays have a pop() method (it works with push()) that reduces the length of an array
by 1 but also returns the value of the deleted element. There is also
a shift() method (which goes with
unshift()) to remove an element
from the beginning of an array. Unlike delete, the shift() method shifts all elements down to
an index one lower than their current index. pop() and shift() are covered in Array Methods and in the reference section.

Finally, splice() is the
general-purpose method for inserting, deleting, or replacing array
elements. It alters the length
property and shifts array elements to higher or lower indexes as
needed. See Array Methods for details.

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