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JavaScript While Loop

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

The While Loop

The while loop executes a specified statement as long as the specified condition evaluates to true.

The while loop has the following syntax:

while (condition) {
    // execute code as long as condition is true
Run code

Note: If you are using a variable with the condition, initialize it before the loop, and increment it within the loop. If you forget to increase the variable, the loop will never end. This will crash your browser.

Infinite Loops

An infinite loop, as the name suggests, is a loop that will keep running forever. If you accidentally make an infinite loop, it could crash your browser or computer. It is important to be aware of infinite loops so you can avoid them.

A common infinite loop occurs when the condition of the while statement is set to true.

An infinite loop will run forever, but the program can be terminated with the break keyword.

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.

The do...while loop has the following syntax:

do {
   // statement(s) to be executed
while (condition);
Run code

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

Note: This 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:

Run code

Difference Between while and do...while Loop

The while loop differs from the do...while loop in one important way — with a while loop, the condition to be evaluated is tested at the beginning of each loop iteration, so if the conditional expression evaluates to false, the loop will never be executed.

With a do...while loop, on the other hand, the loop will always be executed once even if the conditional expression evaluates to false, because unlike the while loop, the condition is evaluated at the end of the loop iteration rather than the beginning.

Comparing For and While Loops

If you have read the previous chapter, about the for loop, you will discover that a while loop is much the same as a for loop, with initialization and final-expression omitted.

The loop in this example uses a for loop to collect the fruit names from the fruits array:

Run code

The loop in this example uses a while loop to collect the fruit names from the fruits array:

Run code