Function to capitalize first three letters on each word within given array

I wrote a function that takes in a single word and capitalizes the first three letters. Now I need to run this same function on an array of words, so that it returns each word with the first three letters capitalized. I see there's a lot of people asking how to capitalize the first letter on each word of a sentence, but this is not the same thing. I have to use the function I already wrote so when I console.log it it looks like this:

console.log(applyAll(['str1', 'str2', 'str3', 'str4'], capitalizeThreeLetters));

I tried doing this using a for loop which returns every word together. In my research I saw the forEach() method could be used to run a function on array elements but I cannot figure out how to apply it.

//Function that takes in str returns it with three first letters capitalized

function capitalizeThreeLetters(str){

  let capFirst = str[0].toUpperCase();
  let capSecond = str[1].toUpperCase();
  let capThird = str[2].toUpperCase();

  let splitStr = str.slice(3);

  let wholeStr = capFirst + capSecond + capThird + splitStr;
  return wholeStr;
}

console.log(capitalizeThreeLetters('testing')); // => returns 'TESting'
console.log(capitalizeThreeLetters('again')); // => returns 'AGAin'



//Function that takes in a string array and applies capitalizeThreeLetters function to each array element so each word is returned with first three letters capitalized

function applyAll(arr){
  
  for (let i = 0; i < arr.length; i++){
  return capitalizeThreeLetters(arr);
  }
}  

console.log(applyAll(['mai', 'brian', 'jeho', 'han'], capitalizeThreeLetters)); 
// => returns 'MAIBRIANJEHOhan'
// => should return ['MAI', 'BRIan', 'JEHo', 'HAN']

Your applyAll function doesn't work the way you want. When you return the function immediately exits. In this case, you return inside the loop thus prevent the loop from continuing and running the other 2 iterations.

Your second issue is that you pass the entire array to capitalizeThreeLetters instead of a single word. You probably want capitalizeThreeLetters(arr[i]). Right now, you are passing the entire array in which leads to the first 3 words in the array being capitalized instead of the first 3 letters of each word.

You can fix it using map:

function applyAll(arr){
  return arr.map(capitalizeThreeLetters);
}

The map function on an array will call a function on each element of the array then return a new array with the results.

Use Array.prototype.map to get a new array with the modified values of a current one

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

//Function that takes in str returns it with three first letters capitalized

function capitalizeThreeLetters(str){

  let capFirst = str[0].toUpperCase();
  let capSecond = str[1].toUpperCase();
  let capThird = str[2].toUpperCase();

  let splitStr = str.slice(3);

  let wholeStr = capFirst + capSecond + capThird + splitStr;
  return wholeStr;
}

const result = ['mai', 'brian', 'jeho', 'han'].map(capitalizeThreeLetters);

console.log(result); 

<!-- end snippet -->

There's no need to convert each [0] [1] etc characters to uppercase, you can use .toUppercase() on the sliced portion.
Also you can parametrize the number of characters to capitalize:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const capitalize = (str, tot=1) => str.slice(0, tot).toUpperCase() + str.slice(tot);

const result = ['mai', 'brian', 'jeho', 'han'].map((str) => capitalize(str, 3));
console.log(result); 

<!-- end snippet -->