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prefer-at

📝 Prefer .at() method for index access and String#charAt().

💼 This rule is enabled in the following configs: ✅ recommended, ☑️ unopinionated.

🔧💡 This rule is automatically fixable by the --fix CLI option and manually fixable by editor suggestions.

Prefer Array#at(), String#at(), and TypedArray#at() for index access and String#charAt().

This rule complements unicorn/prefer-string-slice. This rule handles supported one-character substring() patterns, while unicorn/prefer-string-slice handles general string slicing.

Examples

// ❌
const foo = array[array.length - 1];

// ❌
const foo = array.slice(-1)[0];

// ❌
const foo = array.slice(-1).pop();

// ❌
const foo = array.slice(-1).shift();

// ❌
const foo = lodash.last(array);

// ✅
const foo = array.at(-1);
// ❌
const foo = array[array.length - 5];

// ✅
const foo = array.at(-5);
// ❌
const foo = string.charAt(string.length - 5);

// ✅
const foo = string.at(-5);
// ❌
const foo = string.substring(index, index + 1);

// ✅
const foo = string.at(index);
// ✅
const foo = array[100];
// ✅
// This rule is not checking this case, but `unicorn/prefer-negative-index` rule will fix it.
const foo = array.at(array.length - 1);
// ✅
array[array.length - 1] = foo;
// ✅
// Common DOM collection patterns like `.children`, `.childNodes`, and `querySelectorAll()` are ignored because they are not guaranteed to support `.at()`.
const foo = element.children[element.children.length - 1];

This means non-DOM objects with those exact property or method names are also ignored.

// ✅
// This rule intentionally ignores `arguments`, which is array-like but does not have `Array#at()`.
function foo() {
	return arguments[arguments.length - 1];
}

Options

Type: object

checkAllIndexAccess

Type: boolean
Default: false

This rule only checks negative indexes by default, but you can also check non-negative integer indexes by setting checkAllIndexAccess to true.

Example:

{
	'unicorn/prefer-at': [
		'error',
		{
			checkAllIndexAccess: true
		}
	]
}
/* eslint unicorn/prefer-at: ["error", {"checkAllIndexAccess": true}] */
const foo = bar[10]; // Fails, will fix to `bar.at(10)`
const foo = bar[unknownProperty]; // Passes
const foo = string.charAt(unknownIndex); // Fails

getLastElementFunctions

Type: string[]

You can also check custom functions that get last element of objects.

_.last(), lodash.last(), and underscore.last() are always checked.

Example:

{
	'unicorn/prefer-at': [
		'error',
		{
			getLastElementFunctions: [
				'getLast',
				'utils.lastElement'
			]
		}
	]
}
/* eslint unicorn/prefer-at: ["error", {"getLastElementFunctions": ["utils.lastElement"]}] */
// ❌
const foo = utils.lastElement(bar);
/* eslint unicorn/prefer-at: ["error", {"getLastElementFunctions": ["utils.lastElement"]}] */
// ✅
const foo = bar.at(-1);

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