URL Shortener API

A URL shortener is a common backend coding project.

It tests routing, validation, database design, redirects, and edge cases.

Core routes

POST /urls
GET /:slug
GET /urls/:id/stats

Create short URL

Input:

{
  "longUrl": "https://example.com/some/long/path"
}

Response:

{
  "slug": "aB12xY",
  "shortUrl": "https://short.ly/aB12xY"
}

Data model

const urlSchema = new Schema(
  {
    longUrl: { type: String, required: true },
    slug: { type: String, required: true, unique: true },
    clickCount: { type: Number, default: 0 },
    createdBy: { type: Schema.Types.ObjectId, ref: 'User' },
    expiresAt: Date,
  },
  { timestamps: true },
)

Useful indexes:

urlSchema.index({ slug: 1 }, { unique: true })
urlSchema.index({ createdBy: 1, createdAt: -1 })

Slug generation

Simple version:

function createSlug(length = 6) {
  const chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
  let slug = ''

  for (let i = 0; i < length; i += 1) {
    slug += chars[Math.floor(Math.random() * chars.length)]
  }

  return slug
}

In real code, handle collision by checking whether the slug already exists.

Redirect flow

When user visits:

GET /aB12xY

Server:

1. finds slug in database,
2. increments click count,
3. redirects to long URL.

res.redirect(url.longUrl)

Edge cases

Handle:

• invalid URL,
• duplicate slug,
• missing slug,
• expired links,
• analytics count,
• malicious URLs if required.

Test cases

• creates short URL for valid long URL,
• rejects invalid URL,
• redirects existing slug,
• returns 404 for missing slug,
• increments click count after redirect,
• handles slug collision.

Interview angle

Explain slug generation, uniqueness, redirect route, validation, database schema, and analytics. Mention that high-scale URL shorteners need caching and collision handling.