Loops are very useful for running the same piece of code over and over again.

JavaScript supports different kinds of loops, the two main ones are:

• for - loops through a block of code a number of times.
• while - loops through a block of code while a specified condition is true.

For instance if we were programming a robot to walk forwards 6 steps, we could write the code like this:

stepForward()
stepForward()
stepForward()
stepForward()
stepForward()
stepForward()

Or we could make the code simpler by using a for loop. This will repeat the stepForward() code 6 times.

for(let i = 0; i < 6; i++) {
    stepForward()
}

The for loop takes three statements and then runs whatever code we specify. We put a ; (semicolon) after statement1 and after statement2. This let's the computer know when statement1 and statement2 end.

for(statement1; statement2; statement3)
{
    // repeat this code
}

• statement1 - this code runs once and sets up the variable we will use for our loop.
• statement2 - this is the condition for how many times the loop will run.
• statement3 - this changes the value of the variable in statement1.

Here is an example of a for loop what counts from 1 to 10. You will notice that we can use the variable x inside the for loop.

for(let x = 1; x <=10; x++)
{
    basic.showNumber(x)
}

So for the above example:

• statement1 - let x = 1 creates the variable x and sets it's value to 1.
• statement2 - x <= 10 this is the condition that is checked every time. The loop will keep on repeating as long as x is less than or equal to 10.
• statement3 - x++ this adds 1 to x each time the code is run.

Now let's try writing a for loop. Go to the https://makecode.microbit.org website, create a new project and switch to JavaScript. Try and write a for loop with the following statements:

• statement1 - create a variable called x and set it's value to 1.
• statement2 - specify the condition of x is less than 6.
• statement3 - add 1 to x each time.
• Put basic.showNumber(x) as the code to run in the loop.

see the code

for(let x = 1; x < 6; x++)
{
    basic.showNumber(x)
}

Run the code in the simulator and it should show the numbers 1, 2, 3, 4 and 5. It should not show the number 6 as the condition is less than 6 (not less than or equal to 6).

Now try and write some code to countdown from 10 to 0 using a for loop with the following statements:

• statement1 - create a variable called x and set it's value to 10.
• statement2 - specify the condition of x is greater than or equal than 0.
• statement3 - subtract 1 from x each time.
• Put basic.showNumber(x) as the code to run in the loop.

see the code

for(let x = 10; x >= 0; x--){
    basic.showNumber(x)
}

The while loop takes one statement which is the condition (or conditions) under which the loop should keep on running.

while(condition)
{
    // repeat this code
}

• statement1 - this is the condition (or conditions) that the loop should keep on running until the condition is true.

Here is an example of a while loop what counts from 1 to 10.

let i = 1   // create the variable and set it to 1
while(i <= 10)
{
    basic.showNumber(i) // show i
    i++                 // add 1 to i
}

So for the above example:

• statement1 - i <= 10 this is the condition that is checked every time. The loop will keep on repeating as long as i is less than or equal to 10.