An array built from scratch in JavaScript

An array is one of the first objects a JavaScript developer uses and one of the last things they think about. It supports indexing, iteration, insertion, removal, and a long list of convenience methods. Rebuilding a small version of that behavior is a useful way to see how much is happening behind the familiar syntax.

Choose a small contract

The exercise is more useful when the API is deliberately narrow. Length and emptiness are the starting points. Reading at an index, appending, inserting, prepending, removing from the end, deleting at an index, and searching for a value cover the main mechanics. Each operation has an expected return value, and those expectations are part of the design. A method that mutates the array should say so. A method that returns a new array should not quietly modify the original.

The storage can be a plain object with numeric keys or a normal JavaScript array. Using an object makes the custom behavior more visible because the built in array methods are no longer available. Using an array is closer to how a real implementation would work. Either choice is fine as long as the exercise makes the invariants clear.

Invariants before cleverness

The most important invariant is that the stored items occupy the expected positions after every operation. Inserting in the middle shifts the trailing items to the right. Deleting shifts them to the left. The length must match the number of stored items. Those rules sound obvious, but a single off by one error can leave a hole that only appears when the array is read later.

A second invariant is that a failed operation should not corrupt the structure. Reading an index that does not exist should return a predictable value. Deleting an invalid position should fail without changing the length. The edge cases are where the custom class earns its keep.

Searching and removing values

Searching for a value is linear because every slot may need to be checked. Removing a value can be implemented as a search followed by a delete, which makes the cost easy to reason about. A method that removes every occurrence is a different operation from one that removes only the first. Naming those methods clearly is more valuable than adding another clever overload.

Notebook note: a simple data structure is a collection of small promises. The implementation is trustworthy when those promises hold at the boundaries.

What the exercise reveals

Building the class makes built in arrays feel less like magic and more like a set of carefully maintained behaviors. It also makes the cost of insertion and deletion easier to see. The built in methods are optimized and battle tested, so the custom class is not a replacement. It is a way to understand the tradeoffs that the built in object hides.