fix variable-length array destructuring bug in spend-permissions complete integration example

[weirdDevelop]

Summary

The Complete Integration Example in docs/base-account/improve-ux/spend-permissions.mdx
contains a workflow bug that causes silent runtime failures on repeat executions of a
spend permission.

Problem

Bug 1 — Fixed destructuring of a variable-length array

The guide currently does:

const [approveCall, spendCall] = await prepareSpendCallData({ permission, amount });
calls: [approveCall, spendCall]

According to the prepareSpendCallData reference page, the function returns either 1 or 2
calls depending on whether the permission is already registered onchain:

• First use: returns [approveWithSignatureCall, spendCall] — 2 elements
• Subsequent uses: returns [spendCall] — 1 element

When only 1 element is returned, spendCall becomes undefined. The guide then passes
[approveCall, undefined] directly into wallet_sendCalls, which fails at runtime with no
clear error. The first execution succeeds, every subsequent one silently breaks — making
this extremely hard to diagnose.

Bug 2 — Missing unit context on remainingSpend check

The guide checks:

const { isActive, remainingSpend } = await getPermissionStatus(permission);
if (!isActive || remainingSpend < amount) { ... }

remainingSpend is returned in the token's smallest unit (wei for ETH, 6-decimal units
for USDC). There is no unit guidance anywhere on the page, so a developer comparing
remainingSpend to a human-readable amount value will get incorrect logic for any
ERC-20 token that isn't 18 decimals.

Fix

Bug 1 — Replace fixed destructuring with the full array, matching what the
prepareSpendCallData reference page itself demonstrates:

// Before
const [approveCall, spendCall] = await prepareSpendCallData({ permission, amount });
calls: [approveCall, spendCall]

// After  
const spendCalls = await prepareSpendCallData({ permission, amount });
calls: spendCalls

Bug 2 — Add a one-line inline comment after the remainingSpend comparison clarifying
units:

// remainingSpend and amount are in the token's smallest unit
// (e.g. for USDC with 6 decimals, 1_000_000n = $1.00)

Verification

• prepareSpendCallData return type confirmed against:
  /base-account/reference/spend-permission-utilities/prepareSpendCallData
• getPermissionStatus return shape confirmed against:
  /base-account/reference/spend-permission-utilities/getPermissionStatus
• The reference page's own example uses spendCalls as a whole array — never
  destructured. The guide contradicts its own reference.

Impact

This is a timing-dependent failure. The first execution always succeeds (2-element array,
destructuring works). Every subsequent execution silently fails (1-element array,
spendCall = undefined passed to wallet_sendCalls). Developers who test once and ship
will hit this in production.

Files Changed

• docs/base-account/improve-ux/spend-permissions.mdx

[cb-heimdall]

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