JavaScript while(true): Stop on a Sentinel and Keep a Safety Bound

The task

Read a short command list until END, preserving only commands before the sentinel. Reject missing termination rather than running forever or pretending the whole input was valid.

Runnable Node 22 example

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

function readCommands(input, limit = 5) {
  const accepted = [];
  let index = 0;
  while (true) {
    if (index >= limit || index >= input.length) throw new Error('missing END');
    const command = input[index++];
    if (command === 'END') break;
    accepted.push(command);
  }
  return accepted;
}
assert.deepEqual(readCommands(['scan', 'file', 'END', 'ignored']), ['scan', 'file']);
assert.throws(() => readCommands(['scan']), /missing END/);
assert.throws(() => readCommands(['scan', 'file', 'END'], 2), /missing END/);
console.log(readCommands(['scan', 'file', 'END']).join(','));

Expected output and explanation

The loop checks available input and the configured bound before reading a command. Encountering END exits without appending it; later input is not processed. The second failure shows that a sentinel beyond the allowed reads is not accepted.

The limit is a maximum number of reads including a possible terminator, not a count of accepted work commands. Stating that meaning avoids an off-by-one surprise when a caller chooses the bound.

Pitfall to check

This synchronous example only reads a supplied array. A real asynchronous input source needs appropriate awaiting, cancellation and resource handling rather than a CPU-bound loop waiting for new data.