for loop — JavaScript fundamentals

A while loop works perfectly for situations where the number of iterations isn't known in advance, e.g., finding a prime number. When we know the number of iterations, it's better to use a for loop.

Let's have a look at a function that reverses strings using a for loop:

const reverseString = (str) => {
  let result = '';
  for (let i = 0; i < str.length; i += 1) {
    result = `${str[i]}${result}`;
  }

  return result;
};

We could read it as follows: a loop with index i repeats until i < str.length and after each step increases i by 1.

When defining the for loop, we have three expressions in parentheses, separated by semicolons:

1. The initial counter value. This code runs exactly once before the first iteration
2. A predicate, the condition for the loop to iterate. It's executed on each iteration. Exactly like what we have with while
3. Description of the counter change. This code is executed at the end of each iteration

Other than that, it works exactly the same as the while loop.