How to preflight / preview a transaction?

Question

Is there a way to preflight/preview a transaction before the actual signAndSend?

I would like to know if a transaction would fail before letting user submit the transaction and prevent user losing gas.

I guess polkadot.js has this kind of function, but nothing find in the doc.

Thanks.

Answer 1

You can use the DryRun RPC method to check the outcome of the transaction.

Here is an example:

export async function dryRun(api: ApiPromise, signer: KeyringPair, tx: SubmittableExtrinsic<"promise", ISubmittableResult>): Promise<[boolean, ApplyExtrinsicResult]> {
    const signed = await tx.signAsync(signer);
    const dryRun = await api.rpc.system.dryRun(signed.toHex());
    return [dryRun.isOk && dryRun.asOk.isOk, dryRun]
}

Note that this RPC is marked as unsafe, most public nodes will not expose this and you might need to run your own local node in order to be able to call this.