Looping Over Strings

Remember that strings have a length and each character has an index, starting from 0? That makes them perfect for looping:

const word = "HELLO";

for (let i = 0; i < word.length; i++) {
  console.log(`Index ${i}: ${word[i]}`);
}

Index 0: H
Index 1: E
Index 2: L
Index 3: L
Index 4: O

Notice two important details: we start at i = 0 (the first index), and we use i < word.length, not <=. The word has 5 letters, so the indexes go from 0 to 4. If we used <=, we'd try to read index 5, which doesn't exist.

Counting vowels

Let's count how many vowels are in a sentence:

const sentence = "JavaScript is awesome";
let vowelCount = 0;

for (let i = 0; i < sentence.length; i++) {
  const letter = sentence[i].toLowerCase();
  if ("aeiou".includes(letter)) {
    vowelCount++;
  }
}

console.log(`"${sentence}" has ${vowelCount} vowels.`);

"JavaScript is awesome" has 8 vowels.

That "aeiou".includes(letter) is a neat trick: instead of writing letter === "a" || letter === "e" || ..., we just ask whether the letter appears in a string of all the vowels.

Reversing a string

const original = "stressed";
let reversed = "";

for (let i = original.length - 1; i >= 0; i--) {
  reversed += original[i];
}

console.log(reversed);

desserts

We start at the last index (length - 1) and count down to 0, adding each letter to a new string. "stressed" backwards is "desserts," which is an excellent life lesson.

The for...of loop

When you just want each character and don't care about the index, there's a cleaner loop called for...of:

for (const letter of "WOW") {
  console.log(letter);
}

W
O
W

No counter, no length, no square brackets. Just "for each letter of this string, do this." You'll see a lot more of for...of in Grade 7, when we loop over lists.