After the seller has shipped the goods, he presents a written document to the confirming bank. Therefore, we define the SellerPresentDocument transaction and related event for the LC update:

transaction SellerPresentDocument {
  --> LetterOfCredit lc
  o String evidence
}
event SellerPresentDocumentEvent {
  --> LetterOfCredit lc
} 

The letter status is set to PRESENT DOCUMENT, and the step to 5:

/**
  * seller Presentation the Document
  * @param {org.example.lc.SellerPresentDocument} sellerPresentDocument - seller present document
  * @transaction
  */
async function sellerPresentDocument(request) { // eslint-disable-line no-unused-vars
    const factory = getFactory();
    const namespace = 'org.example.lc';

    let letter = request.lc;

    if (letter.status === 'CLOSED') {
        throw new Error ('This letter of credit has already been closed');
    } else if (letter.step!== 4) {
        throw new Error ('This letter of credit should be in step 4 - ADVICE_LC');
    }
    letter.status = 'PRESENT_DOCUMENT';
    letter.step=5;
    letter.evidence.push(request.evidence);
    const assetRegistry = await getAssetRegistry(request.lc.getFullyQualifiedType());
    await assetRegistry.update(letter);

    // emit event
    const sellerPresentDocumentEvent = factory.newEvent(namespace, 'SellerPresentDocumentEvent');
    sellerPresentDocumentEvent.lc = letter;
    emit(sellerPresentDocumentEvent);
}

The LC asset has an evidence array data field, which contains all the document proofs for the entire process. When a seller presents a written document to the confirming bank, the document will be added to the LC evidence using the JavaScript push function: letter.evidence.push(request.evidence). As you know, the push() method adds new items to the end of an array.