JavaScript For Loop

The for statement creates a loop with 3 optional expressions:

exp 1 is executed (one time) before the execution of the code block.

exp 2 defines the condition for executing the code block.

exp 3 is executed (every time) after the code block has been executed.

From the example above, you can read:

exp 1 sets a variable before the loop starts (let i = 0).

exp 2 defines the condition for the loop to run (i must be less than 5).

exp 3 increases a value (i++) after the code block has been executed.

exp 1 is used to initialize the variable(s) used in the loop (let i = 0).

exp 1 is optional.

You can omit exp 1 if the value is set before the loop starts:

exp 2 is used to evaluate the condition of the initial variable (i < len).

exp 2 is also optional.

If exp 2 returns false , the loop will end.

If you omit exp 2, you must provide a break inside the loop.

Otherwise the loop will never end.

This will crash your browser.

exp 3 increments the value of the initial variable (i++).

exp 3 is optional.

exp 3 can do anything like negative increment (i--), positive increment (i = i + 15), or anything else.

exp 3 can be omitted (if you increment the value inside the loop):

In the second example, using let , the variable declared in the loop does not redeclare the variable outside the loop.

When let is used to declare the i variable in a loop, the i variable will only be visible within the loop.