negativeIndexLengthMethods

Prefer negative index over .length - index for at, slice, splice, and similar methods.

✅ This rule is included in the ts stylisticStrict presets.

Array methods like .at(), .slice(), and .splice() accept negative indices to access elements from the end of an array or string. Using array.slice(array.length - 2) is less readable than array.slice(-2).

Examples

❌ Incorrect

const values = [1, 2, 3];
values.slice(values.length - 2);

const values = [1, 2, 3];
values.slice(values.length - 2, values.length - 1);

const values = [1, 2, 3];
values.at(values.length - 1);

const values = [1, 2, 3];
values.splice(values.length - 1, 1);

const values = [1, 2, 3];
values.with(values.length - 1, 99);

const values = [1, 2, 3];
Array.prototype.slice.call(values, values.length - 2);

✅ Correct

const values = [1, 2, 3];
values.slice(-2);

const values = [1, 2, 3];
values.slice(-2, -1);

const values = [1, 2, 3];
values.at(-1);

const values = [1, 2, 3];
values.splice(-1, 1);

const values = [1, 2, 3];
values.with(-1, 99);

const values = [1, 2, 3];
Array.prototype.slice.call(values, -2);

const values = [1, 2, 3];
values.slice(1, 2);

Options

This rule is not configurable.

When Not To Use It

If you prefer explicit .length - index calculations for documentation purposes, or if you have a codebase convention that requires them for readability in certain contexts, you may want to disable this rule.

Further Reading

• MDN: Array.prototype.at()
• MDN: Array.prototype.slice()
• MDN: Array.prototype.splice()
• MDN: Array.prototype.with()

Equivalents in Other Linters

• ESLint: unicorn/prefer-negative-index