JavaScript forEach: Side Effects, Return Values and Callback Scope

The task

Build a log for dispatch labels while skipping the paused item’s logging. Record every callback visit separately so a skipped append is not mistaken for stopping the iteration.

Runnable Node 22 example

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

const labels = ['print', 'pause', 'pack'];
const visited = [];
const log = [];
const result = labels.forEach(label => {
  visited.push(label);
  if (label === 'pause') return;
  log.push(`dispatch:${label}`);
});
assert.equal(result, undefined);
assert.deepEqual(visited, ['print', 'pause', 'pack']);
assert.deepEqual(log, ['dispatch:print', 'dispatch:pack']);
console.log(log.join(' | '));

Expected output and explanation

The callback returns early for pause, so that label is not appended to log. The later pack callback still runs. visited demonstrates that all three positions reached the callback even though only two produced a dispatch entry.

The method result is undefined rather than a collection of callback results. Use map when the task is producing a transformed array, filter when selecting members, and a control loop when later elements must not run after a condition. Those choices express different tasks rather than interchangeable syntax preferences.

Pitfall to check

forEach also does not collect or await the Promises returned by async callbacks. Choose explicit Promise coordination or an awaited loop when completion order matters; a finished forEach call is not proof the asynchronous work finished.