JavaScript Do While Loop: Exit-Controlled Loops & Guaranteed First Execution

A do-while loop is an exit-controlled loop in JavaScript. Unlike the while loop, the condition is evaluated at the bottom of the loop body. Consequently, the body of a do-while loop is guaranteed to execute at least once, regardless of whether the initial test condition is true or false.

Syntax & Flow Mechanics

// Guaranteed Execution Demo: Condition is FALSE from the start!
let initialScore = 100;

do {
    console.log("This message prints once even though 100 > 500 is false!");
} while (initialScore > 500);

Practical Real-World Example: Interactive Input Validation

Because the user must enter data at least once before you can validate it, do-while is the canonical choice for validation routines:

// Simulating pin-entry retry validation
let attempts = 0;
let pinIsValid = false;

do {
    attempts++;
    // Simulate checking a pin code
    console.log(`Prompting User: Attempt #${attempts}`);
    
    if (attempts === 3) {
        pinIsValid = true; // User typed correct pin on 3rd attempt
    }
} while (!pinIsValid && attempts < 5);

console.log("Access status:", pinIsValid ? "Granted" : "Locked");