9.7. Terminating a Loop With breakΒΆ

JavaScript, like most programming languages, provides a mechanism for terminating a loop before it would complete otherwise. The break keyword, when used within a loop, will immediately terminate the execution of any loop. Program execution then continues at the next line of code below the loop.

Example

This loop executes 12 times, for values of i from 0 to 11. During the twelfth iteration, i is 11 and the condition i > 10 evaluates to true for the first time and execution reaches the break statement. The loop is immediately terminated at that point.

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for (let i = 0; i < 42; i++) {

   // rest of loop body

   if (i > 10) {
      break;
   }

}

The break statement can also be used within a while loop. Consider a situation where we are searching for a particular element in an array. (We have seen that JavaScript has array methods that can carry out array searches, but many programming languages do not.)

We can use a while loop to say, while we have not reached the end of the array, continue iterating. We can then include a break within a conditional check to say, when we have found the element we are searching for, exit the loop.

Example

A while loop can be used with break to search for an element in an array.

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let numbers = [ /* some numbers */ ];
let searchVal = 42;
let i = 0;

while (i < numbers.length) {
   if (numbers[i] === searchVal) {
      break;
   }
   i++;
}

if (i < numbers.length) {
   console.log("The value", searchVal, "was located at index", i);
} else {
   console.log("The value", searchVal, "is not in the array.");
}

Notice that we use a while loop in this example, rather than a for loop. This is because our loop variable, i, is used outside the loop. When we use a for loop in the way we have been, the loop variable exists only within the loop.