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javascript array negative index

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I_Like_Cats__ answered on January 16, 2021 Popularity 9/10 Helpfulness 1/10

answer javascript array negative index

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javascript array negative index

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const arr = ["first", "second", "third"]
console.log(arr[-1]) // Will return undefined

Popularity 9/10 Helpfulness 1/10 Language javascript

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Tags: javascript

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Contributed on Jan 19 2022

I_Like_Cats__

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negative index javascript

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// 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

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Contributed on May 09 2022

MattDESTROYER

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negative indexing in arrays javascript

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// javascript allows negative indexing but not same way as in python
const check = ["orange", "banana", "apple"]
// console.log(check[-1]) //returns an error
check[-1] = "negative fruit"
console.log(check) // ["orange", "banana", "apple", -1: "negative fruit"]
console.log(check[-1]) //returns "negative fruit"
// so it creates a key-value pair

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Tags: arrays indexing javascript

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Contributed on Jan 16 2021

Ranger

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javascript array negative index

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