Why does javascript slice method with negative numbers seem to count differently?

Question

I get the first slice of Banana:

let str = "Apple, Banana, Kiwi";
document.getElementById("demo").innerHTML = str.slice(7,13);

Now this one confuses me because it is zero based and shows this as the answer

let str = "Apple, Banana, Kiwi";
str.slice(-12, -6)    // Returns Banana

not sure why its not " "Banan - quotes mean a space and the -6 above includes the last character but it shouldn't as my example here shows because as javascript states with slice method the last number is not included. Hope you can help the confusion on my part??

I would recommend reading through docs: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… I believe they explain the arguments quite well — zecuria
– zecuria, Commented Sep 2, 2021 at 0:34

DecPK · Accepted Answer · 2021-09-02 00:52:42Z

1

If beginIndex or endIndex is negative then it is treated as

beginIndex = str.length + beginIndex
endIndex = str.length + endIndex

let str = "Apple, Banana, Kiwi";

const negativeStart = -12;
const negativeEnd = -6;

console.log(str.slice(negativeStart, negativeEnd)) // Returns Banana

const start = str.length + negativeStart;
const end = str.length + negativeEnd;
console.log(start);
console.log(end);
console.log(str.slice(start, end));

answered Sep 2, 2021 at 0:52

DecPK

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1 Comment

chris13 Over a year ago

I saw the example and read the Moz and yes it makes sense with the formula you used above and I can see how it works. My error was believing it started from the actual end and counted starting at 0, which seems I am wrong.

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