Update intro-to-anchor-frontend.mdx #735
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The documentation previously showed `new BN(value, "le").toArray()` when demonstrating
memcmp filters. This is misleading because the second argument to `BN` is the base, not
endianness, and the resulting array is variable length.
Updated example here now uses:
new BN(value).toArray("le", 8)
This ensures the number is serialized as an 8-byte little-endian value, consistent with
Rust’s u64 layout on Solana. Without the explicit length, filters may fail when values
require fewer bytes (e.g. 5 → [5] instead of [5, 0, 0, 0, 0, 0, 0, 0]).
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What I did
Corrected BN (Big Number) usage in memcmp filters to ensure proper 8-byte little-endian encoding for
u64valuesProblem
The current documentation shows:
This is problematic for two reasons:
BNis the numeric base, not the endianness. Passing"le"is misleading and may behave inconsistently acrossbn.jsversions..toArray()without an explicit length produces a variable number of bytes. For example,new BN(5).toArray("le")yields[5], but Solana accounts storeu64values in 8-byte little-endian format. Using shorter arrays may still match as a prefix inmemcmpfilters, but it’s unsafe and can produce false positives or fail unexpectedly.Fixes
Updated example to:
Ensures numbers are serialized as fixed-length 8-byte little-endian arrays, consistent with Solana’s on-chain
u64layout. This makesmemcmpfilters deterministic and avoids misleading or unsafe BN usage.Example Failure
Suppose an on-chain account has a
u64field set to65536.On-chain bytes (u64 LE):
Current doc version (
new BN(65536, "le").toArray()):→ Encodes incorrectly, does not match the on-chain value.
Correct version (
new BN(65536).toArray("le", 8)):→ Matches exactly as expected.