Foundations Updated 2026-09 View as Markdown

Loops

for, for...of, for...in and while — and which one to reach for.

The for statement#

JavaScript has two methods for running the same code several times. It is mainly used for iterating over arrays or objects. Let’s see an example:

let i;
for (i = 0; i < 3; i = i + 1)
{
    console.log(i);
}

This will print out the following:

0
1
2

The for statement in JavaScript has the same syntax as in Java and C. It has three parts:

• Initialization - Initializes the iterator variable i. In this example, we initialize i to 0.

• Condition - As long as the condition is met, the loop continues to execute. In this example, we check that i is less than 3.

• Increment - A directive which increments the iterator. In our case, we increment it by 1 on every loop.

We can also write a shorter notation for the statement by inserting the variable definition inside the for loop and incrementing using the ++ operator.

for (let i = 0; i < 3; i++)
{
    console.log(i);
}

To iterate over an array and print out all of its members, we usually use the for statement. Here’s an example:

let myArray = ["A", "B", "C"];
for (let i = 0; i < myArray.length; i++)
{
    console.log("The member of myArray in index " + i + " is " + myArray[i]);
}

This prints out the contents of the array:

The member of myArray in index 0 is A
The member of myArray in index 1 is B
The member of myArray in index 2 is C

Notice that we used the length property of an array, which returns the number of members in the array, so we know when to stop iterating.

The while statement#

The while statement is a more simple version of the for statement which checks if an expression evaluates to true and runs as long as it says true.

For example:

let i = 99;
while (i > 0)
{
    console.log(i + " bottles of beer on the wall");
    i -= 1;
}

break and continue statements#

The break statement allows to stop the execution of a loop. For example, we can create a loop that loops forever using while(true) and use the break statement to break inside the loop instead by checking that a certain condition was met.

let i = 99;
while (true)
{
    console.log(i + " bottles of beer on the wall");
    i -= 1;
    if (i == 0)
    {
        break;
    }
}

The continue statement skips the rest of the loop and jumps back to the beginning of the loop. For example, if we would want to print only odd numbers using a for statement, we can do the following:

for (let i = 0; i < 100; i++)
{
    // check that the number is even
    if (i % 2 == 0)
    {
         continue;
    }
    // if we got here, then i is odd.
    console.log(i + " is an odd number.");
}

let i = 99;
while (true)
{
    console.log(i + " bottles of beer on the wall");
    i -= 1;
    if (i == 0)
    {
        break;
    }
}

for (let i = 0; i < 100; i++)
{
    // check that the number is even
    if (i % 2 == 0)
    {
         continue;
    }
    // if we got here, then i is odd.
    console.log(i + " is an odd number.");
}