mt940-ts

A Typescript library for parsing MT940 messages. It takes as an input text representing a set of MT940 statements and produces an array of statements as output.

Installation

npm install mt940-ts

Usage

The module exports an mt940Parser method that takes a string as an argument and returns an array of statements representing each of the MT940 messages present in the input.

Example

import { readFileSync } from "fs";
import { mt940Parser } from "mt940-ts";

const input = readFileSync("./mt94x.txt").toString();
const parsed = mt940Parser(input);

console.log("Number of statements: ", parsed.length);
for (const statement of parsed) {
  console.log(
    `==== Showing records for account: ${statement.accountIdentification} ====`
  );
  statement.records.forEach((record) => {
    console.log(record);
  });
}

Creating a custom parser

mt940-ts aims to be a simple framework that facilitates the creation of parsers for any MTXXX Swift message. In order to write a custom parser for a given MTXXX message, the following steps must be followed:

• define the tags that compose the MTXXX message
• create an object that represents the structure of the MTXXX message, where each property belongs to a field in the MTXXX message and each value is the tag that corresponds to such property
• create the parser by calling the parserFactory function from src/parser.ts passing the object created on the previous step as the parameter

As an example, lets create a parser for the MT973 message. I chose this one because it's rather small, having only 3 fields. Let's start by extending the src/tags.ts file with the definition of the tag 12 (tags 20 and 25 are already defined since they are used by the MT940 parser):

/*
 * Message type
 *
 * This field identifies the message type which is being requested.
 */
const messageTypeRegex = [RegExp(`^(971|972|998)$`)];

export const tag12 = configTag(
  "12",
  messageTypeRegex,
  (resultList: ParseLineResult[]) => {
    const [{ match, nonMatchReason }] = resultList;

    if (nonMatchReason) throw new Error(nonMatchReason);

    if (!match) throw new Error("Failed to extract message type");

    return { messageType: match[1] };
  }
);

Next, let's create the file src/mt973.ts which will contain the object that defines the structure of the message and it will create and export our parser:

import { tag12, tag20, tag25 } from "./tags";
import { parserFactory } from "./parser";
import { REPEAT } from "./combinators";

const mt973 = {
  transactionReferenceNumber: tag20("mandatory"),
  requestedMessages: REPEAT(tag12("mandatory"), tag25("mandatory")),
};

export const parser = parserFactory(mt973);

Note that the requestedMessages field uses the REPEAT parser combinator combines the given tags into a single tag and expects one or more occurrences of such combined tag in the input.

Now we can import our parser in our project and use it like so:

import { parser as mt973Parser } from "./mt973";

const mt973Msgs = mt973Parser("input");

for (const mt973Msg of mt973Msgs) {
  const { transactionReferenceNumber, requestedMessages } = mt973Msg;
  console.log(transactionReferenceNumber);
  for (const requestedMessage of requestedMessages) {
    console.log(`transaction reference number: ${requestedMessage}`);
    console.log(`message type: ${requestedMessage.messageType}`);
  }
}