Solving the largest possible integer

Our problem is asking us to return the largest possible integer after adding a 5 among the digits. For example, when given the number 268 our expected output is 5268, and when given 670 we are expected to return 6750. When given 0 our output is 50, when given a negative number like -999 our output is expected to -5999.

In order for us to solve this problem, we must first plan where and why we will place our 5. In order for us to have the largest possible integer, we must look at the first integer. If the first integer is less than or equal to five, then in order for the integer to become the largest possible integer, we add the five to the beginning of the number. If the first integer is greater than 5 then we need to add our 5 after the first number. These are our base cases for solving this problem. Now, what about our negative numbers? Well, that is just our base cases flipped.

Here is my code for solving this problem.

const largestPossibleInteger = (N) => {
let arr = []
if (N >= 0) {
arr = String(N).split('')
if (arr[0] <= 5) {
arr.unshift(5)
} else {
for (let i = 0; i < arr.length; i++) {
if (arr[i] > 5) {
continue;
} else {
arr.splice(i, 0, 5)
break;
}
}
}
} else {
let newN = Math.abs(N)
arr = String(newN).split('')
if (arr[0] >= 5) {
arr.unshift(5)
} else {
for (let j = 0; j < arr.length; j++) {
if (arr[j] < 5) {
continue;
} else {
arr.splice(j, 0, 5)
break;
}
}
}
arr.unshift("-")
}
return arr.join('')
}