unshift

Unshift is a term used in programming to describe the operation of adding one or more elements to the beginning of a sequence, typically an array or list. The exact behavior and return value vary by language, but the core idea is to prepend elements to the start while shifting existing elements to higher indices.

In JavaScript, the unshift method is Array.prototype.unshift. It inserts one or more elements at the start of an array, mutating the original array and returning the new length of the array. For example, let a = [2, 3]; a.unshift(1); results in a becoming [1, 2, 3], and the call returns 3.

In Ruby, Array#unshift adds one or more elements to the beginning of the array as well. It mutates the array in place and returns the array itself, so after calling a.unshift(1) on [2, 3], the array becomes [1, 2, 3].

Other languages may offer similar functionality under different names. Python, for example, uses list.insert(0, item) to place an element at the front, while functional languages typically express front insertion via the cons operation (often represented as a new list with the element prepended) rather than a method named unshift.

Performance considerations are common across languages: prepending elements can require shifting existing elements, which is often costlier than appending to the end. When front-insertion is frequent, alternative data structures or strategies may be preferred.

See also: push, shift, insert.