break and continue statements in JS

Exit for loops and iterations prematurely  

Often times a loop is executed until a certain condition evaluates to true, exiting its workload prematurely. In JavaScript, the continue statement is used to exit a foor loop iteration before it concludes, whereas the break statement is used to exit a for loop entirely before it concludes. It's worth pointing out the break and continue statements are applicable to any JavaScript type of loop and not just loops created with the for statement presented earlier.

Listing 6-2 illustrates the use of the break and continue statements in for loops.

Listing 6-2. For loops with break and continue statements

let primeNumbers = [2,3,5,7,11];

const primeLength = primeNumbers.length;

for (let i = 0; i < primeLength; i++) {
  // Continue to next iteration when primeNumber > 4 & < 10
  if (primeNumbers[i] > 4 && primeNumbers[i] < 10) {
     continue;
  }
  console.log(primeNumbers[i]);
}


let vowels = ["a","e","i","o","u"];

const vowelsLength = vowels.length;
console.log("vowels ",vowelsLength);

for (let j = vowelsLength-1; j >= 0; j--) {
  // Break loop when vowel == "i"
  if (vowels[j] == "i") {
     break;
  }
  console.log(vowels[j]);
}

The first example in listing 6-2 loops over the primeNumbers array and inside the block of code checks if the value of the iteration on primeNumbers is larger than 4 or lower than 10 to invoke a continue statement. In this case, when a primeNumbers value is between 4 and 10 the continue statement causes the loop to immediately move to the next iteration, skipping the log statement when the primeNumbers iteration value is 5 and 7.

The second example in listing 6-2 loops over the vowels array with a decreasing counter to facilitate a reverse order loop. Inside the block of code, notice a check is made for when the value of the iteration on vowels equals "i" to invoke a break statement. In this case, when a vowels value equals "i", the condition evaluates to true and the break statement causes the loop to finish immediately, skipping the log statement when the vowels iteration value equals "i", as well as any remaining iterations (i.e. when a vowels value is "e" and "a").