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I was assigned this homework assignment to complete. The question originates from CodeStepByStep. Below is the prompt for the question:

Write a function stutter that takes an array of Strings as a parameter
and that replaces every String with two of that String. For example,
if an array stores the values ["how", "are", "you?"] before the
function is called, it should store the values ["how", "how", "are",
"are", "you?", "you?"] after the function finishes executing.

Below is my implementation:

function stutter(arr) 
 if(arr.length == 0) return []; 
 if(arr.length == 1) 
 arr.push(arr[0]);
 return arr;
 
 let size = arr.length;
 for(let i = 0; i < size + 2; i += 2) 
 arr.splice(i + 1, 0, arr[i]);
 
 //If last two elements are not the same
 if(arr[arr.length - 2] != arr[arr.length - 1] && arr.length != 1) 
 arr.push(arr[arr.length - 1]); 
 
 return arr;

It would be really helpful if I could get some feedback on how my code is written. I am fairly new to JavaScript, and I don't know some of the functions that could have made this a lot easier. Feedback on code efficiency and the implementation itself is warmly invited!

By reviewing your code, the first thing that comes to my mind is a lot of if statements. You should try and keep those minimal by writing solutions to be as general as they can be.

Another thing that I don't like in your code is that you are doing manipulation of array passed to the function, instead of going out with a fresh one. It looks like this manipulation is what leads to a lot of ifs in the first place.

So main point on how you can refactor your code is that you initialize an empty array which will act as a result and then manipulate that code:

function stutter(arr) 
 const result = []

 for (let word of arr) // for..of loop is a bit clearer to read
 result.push(word, word) // push can accept N arguments
 

 return result

So by initalizing resulting array with an empty one, you cleared of a case that the argument array passed in is empty, because for..of loop won't do the looping at all.
I've used for..of loop here since it's less code, but you could also use the C-like for like you've written in your question. Note that that loop also wouldn't loop if the argument array was empty, therefor no need for the if(arr.length == 0).

By the way, I'm a bit puzzled with what exactly is the point of the last if you have in the code, but I think that with the refactoring I've provided, then there is no need for it at all.

Since you've asked for more JS way of doing this, here are two ways:

Using map and flat (this one won't run in Edge):

function stutter(arr) 
 return arr.map(x => [x, x]).flat()

Using just reduce:

function stutter(arr) 
 return arr.reduce((result, current) => [...result, current, current], [])