Отрицательный индекс JavaScript
// Usually negative indexes do not work
const array = [ 1, 2, 3 ];
const string = "123";
console.log(array[-1]); // -> undefined
console.log(string[-1]); // -> undefined
// However arrays and strings have an `at` property which allows negative indexes
// a negative index will work from the end of the array/string backwards:
Indexes: 0 1 2
Array: [ 1, 2, 3 ]
Indexes:-3 -2 -1
/* Array/String.prototype.at essentially acts like this:
Array.prototype.at = String.prototype.at = function(index) {
if (index < 0) {
index = -index;
while (index >= this.length) {
index -= this.length;
}
if (index === 0) {
return this[0];
}
return this[this.length - index];
} else {
while (index >= this.length) {
index -= this.length;
}
return this[index];
}
};
*/
console.log(array.at(-1)); // -> 3
console.log(string.at(-1)); // -> 3
MattDESTROYER