index

Zero-based index of the array element to be returned, converted to an integer. Negative index counts back from the end of the array — if index < 0, index + array.length is accessed.

The element in the array matching the given index. Always returns undefined if index < -array.length or index >= array.length without attempting to access the corresponding property.

The at() method is equivalent to the bracket notation when index is non-negative. For example, array[0] and array.at(0) both return the first item. However, when counting elements from the end of the array, you cannot use array[-1] like you may in Python or R, because all values inside the square brackets are treated literally as string properties, so you will end up reading array["-1"], which is just a normal string property instead of an array index.

The usual practice is to access length and calculate the index from that — for example, array[array.length - 1]. The at() method allows relative indexing, so this can be shortened to array.at(-1).

By combining at() with with(), you can both read and write (respectively) an array using negative indices.

The at() method is generic. It only expects the this value to have a length property and integer-keyed properties.

The following example provides a function which returns the last element found in a specified array.

// Our array with items
const cart = ["apple", "banana", "pear"];

// A function which returns the last item of a given array
function returnLast(arr) {
  return arr.at(-1);
}

// Get the last item of our array 'cart'
const item1 = returnLast(cart);
console.log(item1); // 'pear'

// Add an item to our 'cart' array
cart.push("orange");
const item2 = returnLast(cart);
console.log(item2); // 'orange'

This example compares different ways to select the penultimate (last but one) item of an Array. While all the methods shown below are valid, this example highlights the succinctness and readability of the at() method.

// Our array with items
const colors = ["red", "green", "blue"];

// Using length property
const lengthWay = colors[colors.length - 2];
console.log(lengthWay); // 'green'

// Using slice() method. Note an array is returned
const sliceWay = colors.slice(-2, -1);
console.log(sliceWay[0]); // 'green'

// Using at() method
const atWay = colors.at(-2);
console.log(atWay); // 'green'

The at() method reads the length property of this and calculates the index to access.

const arrayLike = {
  length: 2,
  0: "a",
  1: "b",
  2: "c", // ignored by at() since length is 2
};
console.log(Array.prototype.at.call(arrayLike, 0)); // "a"
console.log(Array.prototype.at.call(arrayLike, 2)); // undefined

• Polyfill of Array.prototype.at in core-js
• Indexed collections guide
• Array
• Array.prototype.findIndex()
• Array.prototype.indexOf()
• Array.prototype.with()
• TypedArray.prototype.at()
• String.prototype.at()