Loops

Loops let a block of code run a certain number of times:

for ( var i = 0; i < 5; i++ ) {
    // Logs "try 0", "try 1", ..., "try 4".
    console.log( "try " + i );
}

Note that in loops, the variable i is not "scoped" to the loop block even though the keyword var is used before the variable name. Scope is covered in more depth in the Scope section.

link The for Loop

A for loop is made up of four statements and has the following structure:

for ( [initialization]; [conditional]; [iteration] ) {
 
    [loopBody]
 
}

The initialization statement is executed only once, before the loop starts. It gives you an opportunity to prepare or declare any variables.

The conditional statement is executed before each iteration, and its return value decides whether the loop is to continue. If the conditional statement evaluates to a falsy value, then the loop stops.

The iteration statement is executed at the end of each iteration and gives you an opportunity to change the state of important variables. Typically, this will involve incrementing or decrementing a counter and thus bringing the loop closer to its end.

The loopBody statement is what runs on every iteration. It can contain anything. Typically, there will be multiple statements that need to be executed, and should be wrapped in a block ({...}).

Here's a typical for loop:

for (var i = 0, limit = 100; i < limit; i++) {
    // This block will be executed 100 times.
    console.log( "Currently at " + i );
    // Note: The last log will be "Currently at 99".
}

link The while loop

A while loop is similar to an if statement, except that its body will keep executing until the condition evaluates to a falsy value.

while ( [conditional] ) {
 
    [loopBody]
 
}

Here's a typical while loop:

var i = 0;
while ( i < 100 ) {
    // This block will be executed 100 times.
    console.log( "Currently at " + i );
    i++; // Increment i
}

Notice that the counter is incrementing within the loop's body. It's possible to combine the conditional and incrementer, like so:

var i = -1;
while ( ++i < 100 ) {
    // This block will be executed 100 times.
    console.log( "Currently at " + i );
}

Notice that the counter starts at -1 and uses the prefix incrementer (++i).

link The do-while Loop

This is almost exactly the same as the while loop, except for the fact that the loop's body is executed at least once before the condition is tested.

do {
 
    [loopBody]
 
} while ( [conditional] )

Here's a do-while loop:

do {
    // Even though the condition evaluates to false
    // this loop's body will still execute once.
    alert( "Hi there!" );
 
} while ( false );

These types of loops are quite rare since only few situations require a loop that blindly executes at least once. Regardless, it's good to be aware of it.

link Breaking and Continuing

Usually, a loop's termination will result from the conditional statement not evaluating to a truthy value, but it is possible to stop a loop in its tracks from within the loop's body with the break statement:

// Stopping a loop
for ( var i = 0; i < 10; i++ ) {
    if ( something ) {
        break;
    }
}

You may also want to continue the loop without executing more of the loop's body. This is done using the continue statement:

// Skipping to the next iteration of a loop
for ( var i = 0; i < 10; i++ ) {
    if ( something ) {
        continue;
    }
 
    // The following statement will only be executed
    // if the conditional "something" has not been met
    console.log( "I have been reached" );
 
}