While Loops

The other common kind of loop in programming languages is the while loop. These loops are defined with an opening assertion, and they execute as long as the condition remains true. Here is an example:

let i = 0;
while (i < 12) {
    console.log(`This is loop iteration number ${i}`);
    i++;
}

In this case, the while loop will execute 12 times, and it will output a message to the web browser's JavaScript console on each loop.

The while loop provides a way to keep executing some set of instructions until a condition changes. This can be powerful, but it can also be dangerous. It's very easy to write an infinite while loop:

while (true) {
    console.log('This is an infinite loop and will lock up your browser tab if you try to run it. (Don\'t do that!)');
}

It might seem insane to have such an easy way to write an infinite loop that could lock up a browser tab, but if used properly, it could work. Imagine we have a system that has a checkValue() function that would give us back a value we are watching for:

while (true) {
    status = checkValue();
    if (status.success == true) {
        break;
    }   
}

In this example, we set up an infinite loop, but once a status is received and checked, it can stop execution using the break command. The break command can be used to stop any loop from executing and return to the execution of code outside the loop. (Please note: This is still probably not the best structure for a program, but it works by way of example.)

Exercises

Please try working these exercises to practice some of the skills we've learned in this section.

let x = 0;
while (x < 43) {
    console.log(`The value of x is: ${x}`);
    x++;
}

assert(x===43, "Incorrect.");