Question 50600 in Javascript Question posted in
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Why does this throw a TypeError? – Javascript

There is a task to find a middle of Linked List. Could you please explain why does fast.next in while() loop throw a "TypeError: Cannot read properties of null (reading ‘next’)"?

const middleNode = (head) => {
    let fast = head
    let slow = head

    if (head !== null) {
        while (fast.next !== null) {
            fast = fast.next.next
            slow = slow.next
        }
    }
    return slow
}
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2

Answers


  1. 0

    Let’s say you have a linked list with two nodes: [1]-[2]

    initial:
fast = [1]
slow = [1]

first iteration (fast.next = [2] != null)
fast = fast.next.next = null
slow = slow.next = [2]

second iteration (attempts to check fast.next = (null.next) => raises TypeError)

    The fix is to change your condition to check both fast && fast.next. You can then remove the if check as well.

    const middleNode = (head) => {
    let fast = head;
    let slow = head;

    while (fast && fast.next) { // fast != null && fast.next != null
        fast = fast.next.next;
        slow = slow.next;
        
    }
    return slow;
}
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  2. 0

    If fast.next doesn’t have a next node fast.next.next will be null therefore on the next iteration when you check

    fast.next !== null

    You are trying to access the next propriety on a null object. You could check if fast exist first like this :

    while (fast!=null && fast.next !== null) {
            fast = fast.next.next
            slow = slow.next
        }
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