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Chrome Extension Collecting Empty Data? Same-Site Cookies with Different Formats Break ID Mapping

· 3 min read
CCLEE
CCLEE

Encountered this issue while building an e-commerce data collection system for a client. Here's the root cause and solution.

TL;DR​

The 1688 platform sets two cookies (last_mid and unb) both containing user IDs, but in different formats (b2b-xxx vs plain digits). The database stores the b2b- prefix format. The original code iterated the cookie array in the outer loop, matching unb first — the strict equality check failed, and all collected data was written with zero values because it couldn't map to a shop.

Fix: Put the cookie key priority list in the outer loop and the cookie array in the inner loop, ensuring high-priority keys are checked first.

Problem​

After collecting daily shop reports from 1688, all dashboard metrics in the database were 0, while inquiry data had values:

shop_id | report_date | reveal_cnt | uv | pay_amt | effective_inq_users
       2 | 2026-05-13  |          0 |  0 |    0.00 |                  56
       2 | 2026-05-14  |          0 |  0 |    0.00 |                  41

Server log: No shop found for memberId: 2214126315258

But the database stored platform_account_id as b2b-2214126315258ad300.

Root Cause​

Two cookies with different formats on the same domain​

unb=2214126315258                          # plain digits
last_mid=b2b-2214126315258ad300            # b2b- prefix + suffix

The shops.platform_account_id column stores the b2b- prefix format. The code used strict equality:

const match = mapping.find(m => m.platform_account_id === memberId);
// "2214126315258" !== "b2b-2214126315258ad300" → no match

Iteration order trap​

The original code had the cookie array in the outer loop and the key list in the inner loop:

// ❌ Wrong: cookie array outer, key priority ineffective
var keys = ['last_mid', '__last_memberid__', 'unb'];
for (var i = 0; i < cookies.length; i++) {     // outer: cookies
    var pair = cookies[i].trim();
    for (var k = 0; k < keys.length; k++) {    // inner: keys
        if (pair.indexOf(keys[k] + '=') === 0) {
            return pair.substring(keys[k].length + 1);
        }
    }
}

document.cookie order is not guaranteed. If unb appears before last_mid in the array, it gets matched first — returning the plain-digit format and making last_mid priority useless.

Solution​

Swap loop levels: key priority list in the outer loop, cookie array in the inner loop:

// ✅ Correct: key priority list in outer loop
var keys = ['last_mid', '__last_memberid__', 'unb'];
for (var k = 0; k < keys.length; k++) {         // outer: iterate keys by priority
    for (var i = 0; i < cookies.length; i++) {   // inner: search all cookies
        var pair = cookies[i].trim();
        if (pair.indexOf(keys[k] + '=') === 0) {
            return pair.substring(keys[k].length + 1);
        }
    }
}

The key list is iterated by priority in the outer loop, so last_mid is always checked first regardless of document.cookie order, guaranteeing the user ID format matches the database.

Verification​

// Confirm both cookies exist in browser console
document.cookie.split(';')
  .filter(c => /last_mid|unb/.test(c.trim()))
  .map(c => c.trim())
// ['unb=2214126315258', 'last_mid=b2b-2214126315258ad300']

Note

This issue doesn't affect chrome.cookies.get() in extension pages — it queries by name directly. But when parsing document.cookie strings, always pay attention to loop levels. Also, if your extension passes data via postMessage, watch out for the targetOrigin wildcard security risk; and if messages are processed twice after hot reload, you'll need to manually manage listener lifecycle.

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