Loop Through a JavaScript Array: Values, Indices and Early Exit

The task

Read work items in order and stop before processing the blocked item. Preserve its zero-based index in a diagnostic message rather than skipping it and pretending later work was reached.

Runnable Node 22 example

const assert = require('node:assert/strict');

const queue = [{id: 'A', ready: true}, {id: 'B', ready: false}, {id: 'C', ready: true}];
const processed = [];
let blockedAt = -1;
for (const [index, item] of queue.entries()) {
  if (!item.ready) { blockedAt = index; break; }
  processed.push(`${index}:${item.id}`);
}
assert.deepEqual(processed, ['0:A']);
assert.equal(blockedAt, 1);
assert.equal(queue.length, 3);
console.log(`${processed.join(',')}; blocked=${blockedAt}`);

Expected output and explanation

Only A reaches the processed list. The check for B runs before the append and breaks the surrounding loop, so C is never inspected by the body afterward. Nothing is removed from the source queue.

entries supplies a pair containing an index and value for each visited position. A plain for...of can omit the index when it is unnecessary. Use an indexed for loop if the algorithm specifically needs to adjust its position. for...in enumerates property keys and is not a substitute for this value-and-control task.

Pitfall to check

Do not use forEach when you need a normal break from traversal. Its callback has separate return behavior. If readiness may change during processing, define whether the loop should use a snapshot or the live state; this fixture has no concurrent updates.