JavaScript Loops

Loops are handy, if you want to run the same code over and over again, each time with a different value.

Often this is the case when working with arrays:

The for statement creates a loop with 3 optional expressions:

exp1 is executed one time before the execution of the code block.

exp2 defines the condition for executing the code block.

exp3 is executed every time the code block has been executed.

exp1 sets a variable before the loop starts (let i = 0).

exp2 defines the condition for the loop to run (i must be less than 5).

exp3 increases a value (i++) each time the code block has been executed.

In the first example, let i = 5; is declared outside the loop.

In the second example, let i = 0; , is declared inside the loop.

When a variable is declared with let or const inside a loop, it will only be visible within the loop.

The while loop loops through a block of code as long as a specified condition is true.

If you forget to increase the variable used in the condition, the loop will never end.

This will crash your browser.

The do while loop is a variant of the while loop.

The do while loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.

The do while runs at least once, even if the condition is false from the start.

This is because the code block is executed before the condition is tested:

Do not forget to increase the variable used in the condition, otherwise the loop will never end!