Hitting an exact KB range in the browser: building an exam photo resizer and age calculator with zero uploads

By Arvind Kumar, Editor, Sarkari Job Selection

Every year, millions of Indian candidates fill in government exam forms. Most of these forms reject your photo unless it meets a narrow spec, for example 200×230 px and 20–50 KB for IBPS bank exams. The usual fix is an "online resizer" that uploads someone's face and signature to a random server. We wanted a version that never uploads anything, so we built one with <canvas> and about 150 lines of plain JavaScript. We also built a small age-eligibility calculator, because "Am I over the age limit?" turns out to be a surprisingly sharp date-math problem.

Here is what was tricky about each one.

Part 1: The photo resizer

Specs differ by exam, so keep them as data

const PRESETS = [ { id: "ibps-photo", w: 200, h: 230, unit: "px", min: 20, max: 50 }, { id: "ibps-sign", w: 140, h: 60, unit: "px", min: 10, max: 20 }, { id: "ssc-sign", w: 6, h: 2, unit: "cm", min: 10, max: 20 }, { id: "upsc-sign", w: 500, h: 500, unit: "px", min: 20, max: 100 }, ]; 

Some notices give sizes in cm, not pixels. We convert using a DPI the user can change (200 by default):

const toPx = (v, unit, dpi) => unit === "cm" ? Math.round(v / 2.54 * dpi) : Math.round(v); 

Only add a preset once you've checked it in the official notification. Where a notice gives only a KB range and no pixel size, leave the pixel fields blank and let the user enter them. Don't guess.

canvas.toBlob has no "target size" option, so binary-search the quality

JPEG size goes down as quality goes down (not perfectly, but closely enough). So we search for the highest quality that still fits under the max:

// encode(W, H, q) -> Promise<ArrayBuffer> async function bestUnderMax(encode, W, H, maxBytes) { let lo = 0.05, hi = 0.95; const top = await encode(W, H, hi); if (top.byteLength <= maxBytes) return { buf: top, q: hi }; const floor = await encode(W, H, lo); if (floor.byteLength > maxBytes) return null; // too many pixels for this budget let best = { buf: floor, q: lo }; for (let i = 0; i < 8; i++) { // 8 steps is plenty const mid = (lo + hi) / 2; const b = await encode(W, H, mid); if (b.byteLength <= maxBytes) { best = { buf: b, q: mid }; lo = mid; } else hi = mid; } return best; } 

The encoder draws onto a white background, so transparent PNGs don't turn black when saved as JPEG:

function encode(img, W, H, q) { const c = document.createElement("canvas"); c.width = W; c.height = H; const x = c.getContext("2d"); x.fillStyle = "#fff"; x.fillRect(0, 0, W, H); x.imageSmoothingQuality = "high"; x.drawImage(img, 0, 0, W, H); // our real code also handles cover/contain return new Promise(r => c.toBlob(b => b.arrayBuffer().then(r), "image/jpeg", q)); } 

If the user didn't fix the pixel size and quality falls below about 50%, we shrink the image by 15% and try again. A slightly smaller, sharp photo looks better than a blocky one.

The weird case: the file is too small

Some forms need a minimum size too (for example "10–20 KB"), and a 140×60 signature can come out at 4 KB. Lowering compression only helps so much. Our fix is to insert a JPEG COM (comment) segment (0xFFFE) right after the SOI marker. Decoders ignore it, and it adds harmless padding bytes:

function padJpeg(buf, minBytes) { const u = new Uint8Array(buf); let need = minBytes - u.length; if (need <= 0) return u; const segs = []; while (need > 0) { const len = Math.min(65533, Math.max(need, 4)); // segment length field max const s = new Uint8Array(len + 2); s[0] = 0xFF; s[1] = 0xFE; s[2] = (len >> 8) & 255; s[3] = len & 255; s.fill(32, 4); segs.push(s); need -= s.length; } const out = new Uint8Array(u.length + segs.reduce((n, s) => n + s.length, 0)); out.set(u.subarray(0, 2), 0); // SOI let off = 2; for (const s of segs) { out.set(s, off); off += s.length; } out.set(u.subarray(2), off); return out; } 

We tell the user when we've padded a file, so nothing happens silently.

Privacy for free

FileReader → Image → canvas → Blob → URL.createObjectURL. The photo never leaves the device, there's no backend to secure, and the tool works on cheap phones. Remember to call URL.revokeObjectURL on the old URL each time you create a new one.

Part 2: The age-limit calculator

A notice says something like "18–27 years as on 01-08-2026". The SSC CHSL 2026 notice spells it out: the candidate must be born not before 02-08-1999 and not later than 01-08-2008. That means:

• you must have reached the minimum age on the cut-off date, and
• you must not have reached the maximum age (so "27" really means "below 27").

const eligible = (age, min, max) => age.y >= min && age.y < max; 

Don't use Date in local time

Timezones and DST will quietly shift dates by one day. Parse YYYY-MM-DD yourself, do the math in UTC, and reject impossible dates like 31-02:

function parse(s) { const m = /^(\d{4})-(\d{2})-(\d{2})$/.exec(s || ""); if (!m) return null; const [y, mo, d] = [+m[1], +m[2], +m[3]]; const dt = new Date(Date.UTC(y, mo - 1, d)); return dt.getUTCMonth() === mo - 1 ? { y, m: mo, d } : null; } const daysIn = (y, m) => new Date(Date.UTC(y, m, 0)).getUTCDate(); function age(b, c) { let months = (c.y - b.y) * 12 + (c.m - b.m); if (c.d < b.d) months--; const ay = b.y + Math.floor((b.m - 1 + months) / 12); const am = (b.m - 1 + months) % 12 + 1; const ad = Math.min(b.d, daysIn(ay, am)); // 29 Feb / 31st clamp const days = Math.round( (Date.UTC(c.y, c.m - 1, c.d) - Date.UTC(ay, am - 1, ad)) / 864e5); return { y: Math.floor(months / 12), m: months % 12, d: days }; } 

Test the boundaries the notice gives you

The notice's own dates make good test cases:

Category relaxation is just max + years (for SSC CGL/CHSL 2026: OBC +3, SC/ST +5). Rules vary by exam, though, so we link every quick preset to its official PDF and tell users to confirm there.

Takeaways

1. Size targets → binary search on quality, then shrink the pixels if quality gets too low.
2. Minimum-size targets → pad with a JPEG COM segment, and tell the user you did.
3. Date eligibility → integer y/m/d math in UTC, and use the notice's own boundary dates as unit tests.
4. Keep specs as data with a source link. Leave out anything you can't verify.

Both tools are free to use (no signup, nothing uploaded): the exam photo & signature resizer and the govt job age calculator. If you know a quality-vs-size heuristic that beats plain binary search, I'd love to hear it in the comments.