Achieving this by split is possible. first create an array of divided strings (by the delimiter ' ') and then loop around the array and capitalize the first char using the method toUpperCase and concat the rest of the string without the first letter using slice

function generateHashtag(str) {
  let split = str.split(' ');
  for (let i = 0; i < split.length; i++) {
    split[i] = split[i].charAt(0).toUpperCase() + split[i].slice(1);
  }
  return split.join('');
}

console.log(generateHashtag('Hello my name is'));

More about split - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split

you can do split[i] = split[i].charAt(0).toUpperCase() + split[i].substring(1) in the loop then outside loop do split.join('')

Basically you are replacing each word (split[i]) with capitalised word. Then in the end join the words.

You don't need to use join at all, just declare and initialize finalResult outside the loop and concatenate each word inside the loop:

function generateHashtag(str) {

    const split = str.split(' '); // Array<String>
    let finalResult = ''; // String

    for(let i = 0; i < split.length; i++) {

        const titleCased = split[i].charAt(0).toUpperCase() + split[i].substring(1);
        finalResult += titleCased;
    }

    return finalResult;
}

console.log(generateHashtag('Hello my name is'));

• However, you can simplify this code considerably by using a functional-programming (FP) style with map and reduce. See below.
• I've also changed your code to use toLocaleUpperCase instead of toUpperCase and uses [0] for brevity.
• It's still safe to use substring(1) for single-character strings, it just returns ''.

function generateHashtag(str) {
    return ( str
        .split(' ')
        .map( word => word[0].toLocaleUpperCase() + word.substring(1).toLocaleLowerCase() )
        .reduce( ( word, concat ) => concat + word, "" )
    );
}

• I forgot that join() can still be used instead of reduce (and will have an optimized implementation inside the JS engine anyway):
• I've also moved the map function's logic to a named function toTitleCase.

function generateHashtag(str) {
    
    const toTitleCase( word ) => word[0].toLocaleUpperCase() + word.substring(1).toLocaleLowerCase();

    return ( str
        .split(' ')
        .map( word => toTitleCase( word ) ) // or just `.map( toTitleCase )`
        .join()
    );
}

The return statement has parens to prevent unwanted automatic-semicolon-insertion which would otherwise break the function.