JS Loop While

JavaScript while & do...while Loops: Condition-Driven Iteration

while and do...while loops execute a code block as long as a specified condition evaluates to true. While while checks the condition before executing the loop body, do...while executes the body at least once before testing the condition.

while vs do...while Flowchart

flowchart TD
    subgraph WhileLoop ["while (condition)"]
    WCond{"Check Condition"} -- "True" --> WBody["Execute Body"] --> WCond
    WCond -- "False" --> WExit["Exit Loop"]
    end

    subgraph DoWhileLoop ["do { ... } while (condition)"]
    DBody["Execute Body (Always 1st Time)"] --> DCond{"Check Condition"}
    DCond -- "True" --> DBody
    DCond -- "False" --> DExit["Exit Loop"]
    end

Loop Characteristics Comparison

Practical Code Example

// Demonstrating while vs do...while loops

// 1. Standard while loop (Polling simulation)
let retryAttempts = 0;
const MAX_RETRIES = 3;
let success = false;

console.log("--- Polling with while loop ---");
while (retryAttempts < MAX_RETRIES && !success) {
  retryAttempts++;
  console.log(`Connection attempt #${retryAttempts}...`);
  if (retryAttempts === 2) {
    success = true;
    console.log("Connection Established!");
  }
}

// 2. do...while loop (Guaranteed first execution)
let inputValid = false;
let attempts = 0;

console.log("
--- Form Validation with do...while loop ---");
do {
  attempts++;
  console.log(`Processing validation attempt #${attempts}`);
  if (attempts >= 1) {
    inputValid = true; // Input validated on first pass
  }
} while (!inputValid);

Best Practices & Gotchas

• Prevent Infinite Loops: Always ensure state variables mutated inside the loop body move toward making the loop condition false.
• Use while for Queue Drain Operations: while (queue.length > 0) is the canonical pattern for processing task queues.
• do...while Semicolon: Remember that do { ... } while (condition); requires a trailing semicolon after the closing parenthesis.

Self-Check Challenge

Write a while loop that continuously divides a number n = 100 by 2 until n becomes less than 10.