a do while loop is a control flow statement that executes a block of code at least once, and then either repeatedly executes the block, or stops executing it, depending on a given boolean condition at the end of the block.
The do while construct consists of a process symbol and a condition. First, the code within the block is executed, and then the condition is evaluated. If the condition is true the code within the block is executed again.
This repeats until the condition becomes false. Because do while loops check the condition after the block is executed, the control structure is often also known as a post-test loop.
Contrast with the while loop, which tests the condition before the code within the block is executed, the do-while loop is an exit-condition loop. This means that the code must always be executed first and then the expression or test condition is evaluated. If it is true, the code executes the body of the loop again.
This process is repeated as long as the expression evaluates to true.
If the expression is false, the loop terminates and control transfers to the statement following the do-while loop. In other words, whereas a while loop sets the truth of a statement as a condition precedent for the code's execution, a do-while loop provides for the action's ongoing execution subject to defeasance by the condition's falsity, which falsity is set as a condition subsequent.
It is possible, and in some cases desirable, for the condition to always evaluate to true, creating an infinite loop. When such a loop is created intentionally, there is usually a break statement that allows termination of the loop.
Syntax
The following statement illustrates the syntax of the do-while loop:
do {
statement(s) to be executed;
} while(expression);
Here is an example
var counter = 0;
do
{
counter++;
console.log('counter is:' + counter);
} while (counter < 5);
You will see the following
counter is:1 counter is:2 counter is:3 counter is:4 counter is:5