One post tagged with "JavaScript"

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']