Writing a smart contract in JavaScript (JS)

Sample contract functionality

The sample contract serves as a demonstration of general QVM logic.

It is capable of:

• registering users into the database

• storing the last registered user's name in the database

• echoing the previous user's name to STDOUT

• incrementing the total registered user count

• storing above counter in the database

Sample code

function main(args) {

    // THIS SAMPLE ONLY SUPPORTS THE "register" FUNCTION
    if (args && args.length === 2 && args[0] === "register") {

        // GET THE CURRENT USER'S NAME OR DEFAULT TO "unknown" IF THIS IS THE FIRST CALL
        const previous_name = process.env.DB_USER_CURRENT || "unknown";

        // GET THE TOTAL USER COUNT
        const total_user_count = parseInt(process.env.DB_TOTALUSERS || "0");

        // WRITE PREVIOUS USER NAME TO STDOUT
        process.stdout.write("OUT=prevname: " + previous_name);
        process.stdout.write("\n");

        // UPDATE CURRENT USER NAME BY WRITING IT TO DB
        process.stdout.write("DBW=USER_CURRENT=" + args[1]);
        process.stdout.write("\n");

        // STORE USER NAME UNDER A STORAGE SLOT FOR PERSISTENCE (CURRENT GETS OVERWRITTEN ON EACH CALL)
        process.stdout.write("DBW=USER_" + total_user_count + "=" + args[1]);
        process.stdout.write("\n");

        // INCREMENT THE TOTAL USER COUNT
        process.stdout.write("DBW=TOTALUSERS=" + (total_user_count + 1));
        process.stdout.write("\n");

        // EXECUTION SUCCESSFUL
        process.exit(0);
    }

    // EXECUTION FAILED, WRONG COMMAND
    if (args.length >= 1) {
        process.stderr.write("Wrong CMD: " + args[0]);
        process.stderr.write("\n");
        process.exit(1);
    }

    // EXECUTION FAILED, WRONG ARGS
    process.stderr.write("Wrong args!");
    process.stderr.write("\n");
    process.exit(1);
}
main(process.argv.slice(2));

Save the contract

Open a text editor and save above sample contract as main.js file.