JavaScript While loop

Loops can execute a block of code as long as a specified condition is true.

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:

                      var i = 0;
                      while (i < 10) {
                        i++;
                        console.log(i);   // Open the console to see the result
                      }
                

Live preview

Warning: 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. This 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);
                  

Example

The example below uses a do/while loop. The loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:

                      do {
                        text += "The number is " + i;
                        i++;
                      }
                      while (i < 10);
                  

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

Exercises - test your knowledge

Exercise 1

Create a while loop that runs as long as i is less than 10.

                        var i = 0;
      
                        while (i < 10) {
                          i++;
                          console.log(i);
                        }
                

Exercise 2

Create a loop that runs as long as i is less than 10, but increase i with 2 each time.

                        var i = 0;
      
                        while (i < 10) {
                          i = i +2;
                          console.log(i);
                        }