Reliable Alternative to .at(-1) for Selecting the Last Item

Overview

Selecting the most recently created object is a common requirement when working with collections. At first glance, using .at(-1) appears to be a straightforward solution: it returns the last element of an array and allows further processing. However, in API handlers and service layers, .at(-1) often becomes unreliable because it depends solely on the current order of elements in the array.

Why .at(-1) Is Not a Reliable Strategy

The order of objects in a store or in-memory structure can change during normal data processing. Filters, object mutations, state updates, and asynchronous operations can alter which object ends up last in the array. .at(-1) does not consider the semantics of the data; it only returns the element at the final index.

A typical scenario involves filtering objects based on a property (for example, status === "created"), and then selecting the last one using .at(-1). If the status of an object changes immediately before filtering—or during the same processing sequence—the result of the filter will no longer contain the expected object. As a result, .at(-1) returns a different element, and new data may be written into the wrong object.

Effects of Reference-Based Mutations

Another limitation stems from how JavaScript handles object references. A filtered list returns references to the original objects. If mutations are applied to these objects while they are simultaneously being filtered or processed, the final selection may not align with the intended logic. This can lead to inconsistent state and unpredictable behavior in the API handler.

A Deterministic Approach Using createdAt

A more stable and predictable method is to select objects based on a timestamp property such as createdAt. Using a temporal attribute removes the dependency on array order and guarantees a deterministic result.

Example:

const lastObj = list.reduce((a, b) =>
  (a.createdAt ?? 0) > (b.createdAt ?? 0) ? a : b
);

This approach consistently selects the object that was truly created last, regardless of how the list has changed during processing. It avoids issues related to status mutations, object updates, or filtering that relies on properties modified earlier in the workflow.

Consistency and Stability in Service Logic

Using createdAt as the primary selection criterion provides a stable and robust mechanism for identifying the last object. This is essential for API handlers that operate on dynamic data, formal state transitions, and structures that are frequently updated.

Adopting this method eliminates the risks associated with .at(-1) and ensures that the system behaves predictably as the implementation evolves and new features are introduced.