JavaScript For Loop

Loops can execute a block of code a number of times.

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:

Different Kinds of Loops

JavaScript supports different kinds of loops:

• for - loops through a block of code a number of times
• for/in - loops through the properties of an object
• for/of - loops through the values of an iterable object
• while - loops through a block of code while a specified condition is true
• do/while - also loops through a block of code while a specified condition is true

The For Loop

The for statement creates a loop with 3 optional expressions:

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

Expression 2 defines the condition for executing the code block.

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

From the example above, you can read:

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

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

Expression 3 increases a value (i++) each time the code block in the loop has been executed.

Expression 1

Normally you will use expression 1 to initialize the variable used in the loop (let i = 0).

This is not always the case. JavaScript doesn't care. Expression 1 is optional.

You can initiate many values in expression 1 (separated by comma):

And you can omit expression 1 (like when your values are set before the loop starts):

Expression 2

Often expression 2 is used to evaluate the condition of the initial variable.

This is not always the case. JavaScript doesn't care. Expression 2 is also optional.

If expression 2 returns true, the loop will start over again. If it returns false, the loop will end.

Expression 3

Often expression 3 increments the value of the initial variable.

This is not always the case. JavaScript doesn't care. Expression 3 is optional.

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

Expression 3 can also be omitted (like when you increment your values inside the loop):

Loop Scope

Using var in a loop:

Using let in a loop:

In the first example, using var, the variable declared in the loop redeclares the variable outside 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.

For/Of and For/In Loops

The for/in loop and the for/of loop are explained in the next chapter.

While Loops

The while loop and the do/while are explained in the next chapters.

let i;
 ( = ;  < ; ) {
  console.log(i);
}