Initial code:

function findLargestNumber(numbers) {
    let largest = 0;
    for (let i = 0; i <= numbers.length; i++) {
        if (numbers[i] > largest) {
            largest = numbers[i];
        }
    }
    return largest;
}

const numbers = [10, 20, 30, 40, 50];
const largestNumber = findLargestNumber(numbers);
console.log("The largest number is: " + largestNumber);

This code has a bug in the loop condition. The loop should iterate until i < numbers.length, but it mistakenly iterates until i <= numbers.length, causing an attempt to access an element outside the array bounds, which results in numbers[i] being undefined. To fix this bug, you should change the loop condition to i < numbers.length:

function findLargestNumber(numbers) {
    let largest = 0;
    for (let i = 0; i < numbers.length; i++) { // Change condition here
        if (numbers[i] > largest) {
            largest = numbers[i];
        }
    }
    return largest;
}

const numbers = [10, 20, 30, 40, 50];
const largestNumber = findLargestNumber(numbers);
console.log("The largest number is: " + largestNumber);

With this fix, the loop will iterate through all elements of the array without attempting to access an element outside the array bounds.