Loops: while and for

We often need to repeat actions.

For example, outputting goods from a list one after another or just running the same code for each number from 1 to 10.

Loops are a way to repeat the same code multiple times.

The “while” loop

The while loop has the following syntax:

while (condition) {
  // code
  // so-called "loop body"
}

While the condition is truthy, the code from the loop body is executed.

For instance, the loop below outputs i while i < 3:

let i = 0;
while (i < 3) { // shows 0, then 1, then 2
  alert( i );
  i++;
}

A single execution of the loop body is called an iteration. The loop in the example above makes three iterations.

If i++ was missing from the example above, the loop would repeat (in theory) forever. In practice, the browser provides ways to stop such loops, and in server-side JavaScript, we can kill the process.

Any expression or variable can be a loop condition, not just comparisons: the condition is evaluated and converted to a boolean by while.

For instance, a shorter way to write while (i != 0) is while (i):

let i = 3;
while (i) { // when i becomes 0, the condition becomes falsy, and the loop stops
  alert( i );
  i--;
}

let i = 3;
while (i) alert(i--);

The “do…while” loop

The condition check can be moved below the loop body using the do..while syntax:

do {
  // loop body
} while (condition);

The loop will first execute the body, then check the condition, and, while it’s truthy, execute it again and again.

For example:

let i = 0;
do {
  alert( i );
  i++;
} while (i < 3);

This form of syntax should only be used when you want the body of the loop to execute at least once regardless of the condition being truthy. Usually, the other form is preferred: while(…) {…}.

The “for” loop

The for loop is more complex, but it’s also the most commonly used loop.

It looks like this:

for (begin; condition; step) {
  // ... loop body ...
}

Let’s learn the meaning of these parts by example. The loop below runs alert(i) for i from 0 up to (but not including) 3:

for (let i = 0; i < 3; i++) { // shows 0, then 1, then 2
  alert(i);
}

Let’s examine the for statement part-by-part:

The general loop algorithm works like this:

Run begin
→ (if condition → run body and run step)
→ (if condition → run body and run step)
→ (if condition → run body and run step)
→ ...

That is, begin executes once, and then it iterates: after each condition test, body and step are executed.

If you are new to loops, it could help to go back to the example and reproduce how it runs step-by-step on a piece of paper.

Here’s exactly what happens in our case:

// for (let i = 0; i < 3; i++) alert(i)

// run begin
let i = 0
// if condition → run body and run step
if (i < 3) { alert(i); i++ }
// if condition → run body and run step
if (i < 3) { alert(i); i++ }
// if condition → run body and run step
if (i < 3) { alert(i); i++ }
// ...finish, because now i == 3

for (let i = 0; i < 3; i++) {
  alert(i); // 0, 1, 2
}
alert(i); // error, no such variable

let i = 0;

for (i = 0; i < 3; i++) { // use an existing variable
  alert(i); // 0, 1, 2
}

alert(i); // 3, visible, because declared outside of the loop

Skipping parts

Any part of for can be skipped.

For example, we can omit begin if we don’t need to do anything at the loop start.

Like here:

let i = 0; // we have i already declared and assigned

for (; i < 3; i++) { // no need for "begin"
  alert( i ); // 0, 1, 2
}

We can also remove the step part:

let i = 0;

for (; i < 3;) {
  alert( i++ );
}

This makes the loop identical to while (i < 3).

We can actually remove everything, creating an infinite loop:

for (;;) {
  // repeats without limits
}

Please note that the two for semicolons ; must be present. Otherwise, there would be a syntax error.

Breaking the loop

Normally, a loop exits when its condition becomes falsy.

But we can force the exit at any time using the special break directive.

For example, the loop below asks the user for a series of numbers, “breaking” when no number is entered:

let sum = 0;

while (true) {

  let value = +prompt("Enter a number", '');

  if (!value) break; // (*)

  sum += value;

}
alert( 'Sum: ' + sum );

The break directive is activated at the line (*) if the user enters an empty line or cancels the input. It stops the loop immediately, passing control to the first line after the loop. Namely, alert.

The combination “infinite loop + break as needed” is great for situations when a loop’s condition must be checked not in the beginning or end of the loop, but in the middle or even in several places of its body.

Continue to the next iteration

The continue directive is a “lighter version” of break. It doesn’t stop the whole loop. Instead, it stops the current iteration and forces the loop to start a new one (if the condition allows).

We can use it if we’re done with the current iteration and would like to move on to the next one.

The loop below uses continue to output only odd values:

for (let i = 0; i < 10; i++) {

  // if true, skip the remaining part of the body
  if (i % 2 == 0) continue;

  alert(i); // 1, then 3, 5, 7, 9
}

For even values of i, the continue directive stops executing the body and passes control to the next iteration of for (with the next number). So the alert is only called for odd values.

for (let i = 0; i < 10; i++) {

  if (i % 2) {
    alert( i );
  }

}

if (i > 5) {
  alert(i);
} else {
  continue;
}

(i > 5) ? alert(i) : continue; // continue isn't allowed here

Labels for break/continue

Sometimes we need to break out from multiple nested loops at once.

For example, in the code below we loop over i and j, prompting for the coordinates (i, j) from (0,0) to (2,2):

for (let i = 0; i < 3; i++) {

  for (let j = 0; j < 3; j++) {

    let input = prompt(`Value at coords (${i},${j})`, '');

    // what if we want to exit from here to Done (below)?
  }
}

alert('Done!');

We need a way to stop the process if the user cancels the input.

The ordinary break after input would only break the inner loop. That’s not sufficient – labels, come to the rescue!

A label is an identifier with a colon before a loop:

labelName: for (...) {
  ...
}

The break <labelName> statement in the loop below breaks out to the label:

outer: for (let i = 0; i < 3; i++) {

  for (let j = 0; j < 3; j++) {

    let input = prompt(`Value at coords (${i},${j})`, '');

    // if an empty string or canceled, then break out of both loops
    if (!input) break outer; // (*)

    // do something with the value...
  }
}

alert('Done!');

In the code above, break outer looks upwards for the label named outer and breaks out of that loop.

So the control goes straight from (*) to alert('Done!').

We can also move the label onto a separate line:

outer:
for (let i = 0; i < 3; i++) { ... }

The continue directive can also be used with a label. In this case, code execution jumps to the next iteration of the labeled loop.

break label; // jump to the label below (doesn't work)

label: for (...)

label: {
  // ...
  break label; // works
  // ...
}

Summary

We covered 3 types of loops:

• while – The condition is checked before each iteration.
• do..while – The condition is checked after each iteration.
• for (;;) – The condition is checked before each iteration, additional settings available.

To make an “infinite” loop, usually the while(true) construct is used. Such a loop, just like any other, can be stopped with the break directive.

If we don’t want to do anything in the current iteration and would like to forward to the next one, we can use the continue directive.

break/continue support labels before the loop. A label is the only way for break/continue to escape a nested loop to go to an outer one.