Expose a Rust const to TypeScript clients: #[constant] makes Anchor emit it in the IDL

Problem: a program declares pub const ABC: u16 = 1; (or a seed literal, or an admin pubkey) and the client wants that exact value without duplicating it in TypeScript. A plain pub const inside #[program] does not appear anywhere in the generated IDL or in the generated types file, so idl.constants is empty and there is nothing to import.

Fix: annotate the constant with Anchor's #[constant] attribute. This requires the newer IDL spec, i.e. anchor-lang / Anchor CLI 0.30 or later.

#[constant]
pub const ADMIN_ADDRESS: Pubkey = pubkey!("EA4ptgF3TYjDBGYJApAoZoyCbCYw6P5mGU5noCe1Z97");

After anchor build, the IDL JSON gains a top-level constants array alongside types:

"constants": [
  { "name": "ADMIN_ADDRESS", "type": "pubkey", "value": "EA4ptg..." },
  { "name": "SEED_CUSTOM_SEED", "type": "bytes", "value": "[83, 101, 105, 107, 97]" }
]

Client code then reads idl.constants (byte arrays come through as a stringified array you must parse, and pubkeys as base58 strings). This is the supported way to keep PDA seed literals in exactly one place instead of hand-copying them into the frontend, where a typo produces a wrong PDA and a confusing constraint failure at runtime.

Non-obvious part: the feature is opt-in per constant and silent when missing — nothing warns you that your constant was skipped, so the natural conclusion is that Anchor cannot export constants at all.

Source: adapted from a Stack Exchange answer by Jimii (question by Eric), https://solana.stackexchange.com/questions/20541/anyway-to-access-programs-const-value-via-idl-or-types-in-web3 — CC BY-SA 4.0