Question 168558 in Javascript Question posted in
A very good W3school tutorial can be found here.

“Can anyone help me understand why I’m getting an Uncaught TypeError when using this Javascript code?”

let friends = ["Ahmed", "Sayed", "Ali", 1, 2, "Mahmoud", "Amany"];
let index = 0;
let counter = 0;

// Output
"1 => Sayed"
"2 => Mahmoud"

while (index < friends.length) {
    index++
    if (typeof friends[index] === "number") {
        continue;
    }
    if (friends[index].startsWith(friends[counter][counter])) {
      continue;
    }
    console.log(friends[index])
}

And i Expect to get this output.

"1 => Sayed"
"2 => Mahmoud"

but console gave me this error Uncaught TypeError: Cannot read properties of undefined (reading ‘startsWith’)

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2

Answers


  1. 0

    Try adding friends[index] to your check. Also, your loop condition should be friends.length – 1

    let friends = ["Ahmed", "Sayed", "Ali", 1, 2, "Mahmoud", "Amany"];
let index = 0;
let counter = 0;

while (index < friends.length -1) {
    index++
    if (typeof friends[index] === "number") {
        continue;
    }
    if (friends[index] && friends[index].startsWith(friends[counter][counter])) {
      continue;
    }
    console.log(friends[index])
}
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  2. 0

    Your problem is, that you increment the index at the start of the loop instead of at the end. This means, that when you are at index 6 it still satisfys the while condition and then in the loop gets incremented to 7 which is outofbounds for the friends Array. My solution would be to switch from a while loop to a for loop.

    let friends = ["Ahmed", "Sayed", "Ali", 1, 2, "Mahmoud", "Amany"];
let counter = 0;

for (let index = 0; index < friends.length; index++) {
    if (typeof friends[index] === "number") {
        continue;
    }
    if (friends[index].startsWith(friends[counter][counter])) {
      continue;
    }
    console.log(friends[index])
}
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