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 — ifindex < 0,index + array.length is accessed.

The element in the array matching the given index. Always returnsundefined ifindex < -array.length orindex >= array.length without attempting to access the corresponding property.

Theat() method is equivalent to the bracket notation whenindex is a non-negative integer. For example,array[0] andarray.at(0) both return the first item. However, when counting elements from the end of the array, you cannot usearray[-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 readingarray["-1"], which is just a normal string property instead of an array index.

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

By combiningat() withwith(), you can both read and write (respectively) an array using negative indices.

Theat() method isgeneric. It only expects thethis value to have alength property and integer-keyed properties.

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

// Our array with itemsconst cart = ["apple", "banana", "pear"];// A function which returns the last item of a given arrayfunction 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' arraycart.push("orange");const item2 = returnLast(cart);console.log(item2); // 'orange'

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

// Our array with itemsconst colors = ["red", "green", "blue"];// Using length propertyconst lengthWay = colors[colors.length - 2];console.log(lengthWay); // 'green'// Using slice() method. Note an array is returnedconst sliceWay = colors.slice(-2, -1);console.log(sliceWay[0]); // 'green'// Using at() methodconst atWay = colors.at(-2);console.log(atWay); // 'green'

Theat() method reads thelength property ofthis 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 ofArray.prototype.at incore-js
• es-shims polyfill ofArray.prototype.at
• Indexed collections guide
• Array
• Array.prototype.findIndex()
• Array.prototype.indexOf()
• Array.prototype.with()
• TypedArray.prototype.at()
• String.prototype.at()