As you say, the job of the splice method is to modify the contents of the array. The two args you're passing it are the start position and the delete count. You also need the replacement value, and to keep replacing until that condition is not longer true, so the while loop is also an option:

let nums = [0, 0, 1, 1, 1, 2, 2, 3, 3, 4];

var removeDuplicates = (nums) => {
  for (let i = 0; i < nums.length; i++) {
    while (nums[i] === nums[i + 1]) {
      nums.splice(i, 2, nums[i]);
    }
  }
};

removeDuplicates(nums);

console.log(nums);

Output:

[ 0, 1, 2, 3, 4 ]

The first problem is here: nums.splice(i, i + 1);

Array splice method takes 2 arguments, the first one is the index of element, and the second one is a number of elements to replace/remove (it seems you confused the second argument with the end index). Source: Array.prototype.splice()

Another problem is changing the original array, while iterating over it (be very careful with such in-place manipulations, they very often result in some kind of errors). Look at what is happening in your loop:

1. You have some array [0, 0, 0, 1, 1, 2, 2];
2. You want to remove the first element, since it is a duplicate: [0, 0, 0, 1, 1, 2, 2];
3. After removing, you have [0, 0, 1, 1, 2, 2];
4. You increment your i, so now it is equal to 1;
5. You look at the element with index 1, and find no duplicates [0, 0, 1, 1, 2, 2], since you just skipped the element, that moved into the first index in place of the removed element.

The solution is to decrement the i after removing the element:

let nums = [0, 0, 1, 1, 1, 2, 2, 3, 3, 4];

var removeDuplicates = (nums) => {
  for (let i = 0; i < nums.length; i++) {
    if (nums[i] === nums[i + 1]) {
      nums.splice(i, 1);
      i--;
    }
  }
};

removeDuplicates(nums);

console.log(nums);
// [0, 1, 2, 3, 4]