JavaScript startsWith: Check a Prefix at the Intended Position

The task

Inspect a fictional dispatch reference and the same reference inside a labeled message. Then compare a string that merely contains the prefix later.

Runnable Node 22 example

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

const reference = 'JOB-314';
assert.equal(reference.startsWith('JOB-'), true);
assert.equal(reference.startsWith('job-'), false);
assert.equal('Ref: JOB-314'.startsWith('JOB-', 5), true);
assert.equal('old-JOB-314'.startsWith('JOB-'), false);
assert.equal('old-JOB-314'.includes('JOB-'), true);
assert.equal('JOB-'.startsWith('JOB-'), true);
console.log(`prefix=${reference.startsWith('JOB-')}; nested=${'old-JOB-314'.startsWith('JOB-')}`);

Expected output and explanation

The offset five points immediately after “Ref: ” in the labeled message. The old-reference example contains the marker but fails the start check, which shows why includes would accept a broader set of inputs.

The bare JOB- string also passes the prefix test even though it has no number. If the complete reference requires digits, a length or grammar check must enforce that part. Prefix checking alone cannot prove the suffix is present or valid.

Pitfall to check

Do not silently lowercase a case-sensitive reference before checking it. Decide whether the format permits that normalization. Position is a string offset, not a record field number, so confirm it against the actual input layout.