append.js

export const append = (demo = '', file = '') => {
  const params = new URLSearchParams({
    ...(file ? { file } : {}),
    ctl: 1,
    embed: 1,
  })
  return `/${demo}?${params}`
}

append.test.js

import { append } from './main.js'

import test from 'node:test'
import assert from 'node:assert/strict'

test('handles having both demo and file', () => {
  const out = append('demo', 'file')
  assert.strictEqual(out, '/demo?file=file&ctl=1&embed=1')
})

test('handles only demo', () => {
  const out = append('demo', undefined)
  assert.strictEqual(out, '/demo?ctl=1&embed=1')
})

test('handles only file', () => {
  const out = append(undefined, 'file')
  assert.strictEqual(out, '/?file=file&ctl=1&embed=1')
})

test('handles missing demo and file', () => {
  const out = append(undefined, undefined)
  assert.strictEqual(out, '/?ctl=1&embed=1')
})