Difference between for and for..of loops' output in a specific example

July 18, 2022

I have here two snippets of code, one using traditional for loop and the other using for…of loop.

export function spinWords(words: string): string {
  const singleWords: string[] = words.split(' ');
  for (let i = 0; i < singleWords.length; i++) {
    if (singleWords[i].length >= 5) {
    singleWords[i] = singleWords[i].split('').reverse().join('');
    }
  }
  return singleWords.join(' ');
}


export function spinWords(words: string): string {
   const singleWords: string[] = words.split(' ');
   for (let word of singleWords) {
     if (word.length >= 5)  {
       word = word.split('').reverse().join('');
     }
   }
   return singleWords.join(' ');
}

Why is that in the former snippet, my function spinWords was able to reverse any word with a length >5, whereas in the latter, the array singleWords remain unchanged even the word elements (with length>=5) were reversed?

>Solution :

With the for loop you are manipulating the entry in the array, while with the for of loop you get a new variable word (which is correctly reversed) but only for the variable itself. You are not changing anything in the array. if you do:

export function spinWords(words: string): string {
   const singleWords: string[] = words.split(' ');
   for (let word of singleWords) {
     if (word.length >= 5)  {
       const idx = singleWords.indexOf(word);  
       singleWords[idx] = word.split('').reverse().join('');
     }
   }
   return singleWords.join(' ');
}

It will work the same as the for loop.

Playground

NOTE: while this works, its better to use a for loop (or using the built in forEach, that can access the index as well) for this use case here.