JS Loops

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:

Instead of writing:

  text += cars[0] + "<br>";
text += cars[1] + "<br>";

 text += cars[2] + "<br>";
text += cars[3] + "<br>";

 text += cars[4] + "<br>";
text += cars[5] + "<br>";

The For Loop

The for statement creates a loop with 3 optional expressions:

for (expr1; expr2; expr3) {

      // code block to be executed

 }

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.

Example

for (let i = 0; i < 5; i++) {

   text += "The number is " + i + "<br>";

}

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.

Loop Scope

Example

let i = 5;

for (i = 0; i < 10; i++) {

  // some code
}

// Here i is 10

Example

let i = 5;

for (let i = 0; i < 10; i++) {

    // some code
}

// Here i is 5

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

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

Syntax

while (condition) {

  // code block to be executed

 }

Example

In the following example, the code in the loop will run, over and over again, as long as a variable (i) is less than 10:

Example

while (i < 10) {

    text += "The number is " + i;

    i++;
}

Note: 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

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.

Syntax

do {

// code block to be executed

}

while (condition);

Note: 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:

Example

do {

    text += "The number is " + i;

    i++;

 }

while (i < 10);

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