shift and unshift

Learn to add/remove at the start with .shift and .unshift.

Prepending with unshift

To add an element at the start of an array you use .unshift(x). arr.unshift(0) places 0 at the start of arr and returns the new length. Existing elements shift one position to the right.

For example after const nums = [2, 3]; nums.unshift(1);, nums becomes [1, 2, 3]. unshift is the start-side counterpart of push. Several values can be added: arr.unshift(-1, 0) prepends both. unshift also modifies the array in place.

Removing from the start with shift

To remove the first element of an array you use .shift(). It returns the removed element and the remaining elements shift one position left. In const nums = [1, 2, 3]; const first = nums.shift();, first becomes 1 and nums is left as [2, 3].

Calling shift on an empty array returns undefined. shift and unshift together give FIFO (first in first out) queue behavior. If you append with push and remove with shift you get a queue. shift also modifies the array in place.

Exercise Prepend with unshift and print

The nums variable is set to [2, 3]. Prepend 1 with nums.unshift(1); then print nums[0] with console.log. The output on a single line must be 1.

Show example solution

const nums = [2, 3];
nums.unshift(1);
console.log(nums[0]);

Test yourself

What is the result of const a = [2, 3]; a.unshift(1);?
```
const a = [2, 3];
a.unshift(1);
```
- a = [2, 3, 1]
- a = [1]
- a = [2, 3]
- a = [1, 2, 3]
Show answer

a = [1, 2, 3] — unshift prepends 1; a becomes [1, 2, 3].
Match each method to its job

unshift

Prepends at start
shift

Removes from start
push

Appends at end
pop

Removes from end

Write the shift call

const nums = [1, 2, 3];
const first = nums____();

Show answer

const nums = [1, 2, 3];
const first = nums.shift();

Next lesson: splice and slice

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