Parse a page selection completely or not at all

A page-selection field looks small until you ask what should happen with 1,, 2junk, or 1-1000,1. A forgiving parser might quietly keep the part it understands. For a document operation, that can leave the user with a different selection from the one they meant.

This example takes a narrower approach: under an explicit teaching policy, it accepts the entire input or returns null. It converts displayed page numbers, which start at one, into zero-based indices. It does not open or modify a PDF.

Define the contract before parsing

The grammar accepts ASCII positive integers and inclusive ranges separated by commas. Whitespace is allowed around tokens but not inside a range. Descending ranges are valid. If a page occurs more than once, the output keeps its first occurrence, so 4-2,2,6 becomes [3,2,1,5] for a six-page document.

The policy rejects leading zeroes, full-width digits, semicolons, empty tokens, trailing text, and out-of-range pages. Those choices help only if the interface tells users what it accepts. They are not a universal definition of page selection, and they do not describe a production parser's complete grammar.

The limits are part of this example's contract: at most 120 UTF-16 code units, 20 tokens, and 1,000 expanded occurrences, with a page count between 1 and 10,000. Expansion is counted before deduplication, so 1-1000,1 exceeds the budget even though the unique result would contain only 1,000 pages. A selection can be syntactically valid and still fail the budget.

export function parsePages(text, pageCount) {
  if (typeof text !== 'string' || text.length > 120 ||
      !Number.isSafeInteger(pageCount) || pageCount < 1 || pageCount > 10000) return null;
  const tokens = text.trim().split(',');
  if (tokens.length > 20) return null;
  const pages = [], seen = new Set();
  let expanded = 0;
  for (const token of tokens) {
    const match = /^([1-9]\d*)(?:-([1-9]\d*))?$/.exec(token.trim());
    if (!match) return null;
    const from = Number(match[1]), to = Number(match[2] ?? match[1]);
    if (!Number.isSafeInteger(from) || !Number.isSafeInteger(to) ||
        from > pageCount || to > pageCount) return null;
    expanded += Math.abs(to - from) + 1;
    if (expanded > 1000) return null;
    const step = to >= from ? 1 : -1;
    for (let page = from; page !== to + step; page += step) {
      if (!seen.has(page)) { seen.add(page); pages.push(page - 1); }
    }
  }
  return pages;
}

Check the policy, including rejections

Paste the assertions below after the function in the same .mjs file. The tests compare complete arrays, so they exercise both the indexing and the first-occurrence order. They also make the rejection policy visible to anyone who changes the parser later.

import assert from 'node:assert/strict';
const valid = [
 ['1-3,5',6,[0,1,2,4]],
 ['4-2,2,6',6,[3,2,1,5]],
 [' 1 , 3-2 ',3,[0,2,1]],
 ['2,2',3,[1]],
 ['1-1000',1000,Array.from({length:1000},(_,i)=>i)]
];
for (const [text,count,expected] of valid) assert.deepEqual(parsePages(text,count),expected);
const invalid = [
 ['',3],['1,',3],['2junk',3],['0',3],['4',3],['01',3],['１',3],
 ['1;2',3],['1 - 2',3],['1-1001',1001],['1-1000,1',1000],
 ['1',0],['1',2.5],['1',10001],[null,3],['1'.repeat(121),3],
 [Array(21).fill('1').join(','),3]
];
for (const [text,count] of invalid) assert.equal(parsePages(text,count),null);
console.log(JSON.stringify({valid:valid.length,rejected:invalid.length,boundary:'synthetic text and page indices only'}));

Number.isSafeInteger guards the numeric inputs and the parsed endpoints, and a Set records pages already emitted. Neither one decides how an application should report a rejection. This function returns the same value for every invalid case, so a user interface that needs specific explanations would need a separate, carefully defined diagnostic contract.

The saved November 1 Node v22.23.4 run records five valid and 17 rejected synthetic cases. The stub exited with code 1 and the implementation with code 0. That evidence covers these text and index examples, not PDF decoding or writing, ZIP creation, browser or device behavior, or conversion.

Disclosure: Drafted with GPT-6.1 Terra for Yan Wang / LensUp; Yan Wang is affiliated with LensUp. Polished by an independent editorial AI (lensup 主编); Codex (AI) fact-checked the text and checked the code against the retained synthetic runs. No human editor or owner reviewed or tested it. The background is November 1 source and public-page inspection plus the saved synthetic Node run described above.