Undefined when reading length with nested for loops in JAVA SCRIPT

Question

let arr=[[1,2,3],[4,5,6],[7,8,9]];

for (let i=arr.length;i>=0;i--){
  console.log(arr[i]);
  for (let n=arr[i].length;n>=0;n--){
    console.log(arr[i][n]);
  }
}

arr.length is 3 in your example. Which means that only arr[0],arr[1] and arr[2] have a value. Your for loop correctly includes 0 (condition is i>=0, not i<0), but incorrectly starts with i=3, which is not a valid index — chrslg
– chrslg, Commented Oct 1, 2022 at 17:37

Rayon · Accepted Answer · 2022-10-01 17:38:11Z

1

You are trying to read out of range of arrays. Add -1

let arr = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
];

for (let i = arr.length - 1; i >= 0; i--) {
  console.log(arr[i]);
  for (let n = arr[i].length - 1;/* While accessing index, always consider `length - 1` */ n >= 0; n--) {
    console.log(arr[i][n]);
  }
}

edited Oct 1, 2022 at 17:38

Rayon

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answered Oct 1, 2022 at 17:35

Konrad

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2 Comments

Yusuf Khan Over a year ago

thank you so much brother....i am learning the basics of JS ....but i have stuck with this problem since 2 hours .....but you solved it ..thank u thank u ...

Konrad Over a year ago

I would personally ask a question here only after days of research meta.stackoverflow.com/questions/261592/…

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