while / do...while

JavaScript · Reference cheat sheet

📋 Overview

while repeats while a condition is truthy. do...while runs the body at least once. Use when the end condition is not a simple index range — and always ensure progress toward termination.

🔧 Core concepts

while (cond) \{ body \}: test first.
do \{ body \} while (cond): test after.
Control: break, continue, labels.
Condition: coerced with ToBoolean.
vs for: for is clearer for known bounds; while for sentinel / stream / backoff loops.

let n = 3;
while (n > 0) {
  console.log(n);
  n -= 1;
}

💡 Examples

// Read until sentinel
let line;
while ((line = readLine()) !== null) {
  process(line);
}

// do...while menu
let choice;
do {
  choice = prompt("1) go  0) quit");
} while (choice !== "0" && choice !== null);

// Exponential backoff
async function retry(fn, attempts = 5) {
  let i = 0;
  let delay = 100;
  while (true) {
    try {
      return await fn();
    } catch (err) {
      if (++i >= attempts) throw err;
      await new Promise((r) => setTimeout(r, delay));
      delay *= 2;
    }
  }
}

// Linked list walk
let node = head;
while (node) {
  visit(node);
  node = node.next;
}

// continue / break
let i = 0;
while (i < 10) {
  i += 1;
  if (i % 2 === 0) continue;
  if (i > 7) break;
  console.log(i);
}

// Poll with abort
async function waitFor(pred, signal, ms = 50) {
  while (!signal.aborted) {
    if (await pred()) return true;
    await new Promise((r) => setTimeout(r, ms));
  }
  return false;
}

⚠️ Pitfalls

Infinite loops from missing updates or always-true conditions — guard with counters/timeouts.
while (await fetch...) without delay can hammer servers.
Assignments in conditions (while (x = next())) need parentheses and careful null checks.
continue in do...while still re-checks the condition — know your flow.
Prefer for...of when iterating collections.

iteration.md — loop overview
for_of.md — for...of
boolean.md — truthiness
async.md — async loops
promise.md — delays / retry
iterator.md — generator loops

while / do...while

while / do...while

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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