why yargs.argv causes handler function to invoke

const yargs = require("yargs");

yargs.version = "1.0.0";

yargs.command({
  command: "add",
  describe: "do addition ",
  handler: function () {
    console.log("addition complete");
  },
}).argv;

In the above code using yargs i was trying to create a command add and on running the script with node . add the handler function is executed and i get the the expected output

$ node . add
addition complete

but when the same code is run without the .argv the output is blank and the handler function is not executed . in the docs it is not stated what .argv is , weather its an object or a function and how it actually works . at first glance it looks like a normal method invocation but

When you do the following
console.log(typeof yargs.argv);

The output is

$ node . add
addition complete
object

If yargs.argv is an like any other Object then just by accessing it how and why it is able to invoke a function

can somebody explain whats actually happening and where am i missing out .

Answer

Yargs is using a #getter, this allows javascript to call a function when a property is called.

Example:

const obj = {
  log: ['a', 'b', 'c'],
  get latest() {
    if (this.log.length === 0) {
      return undefined;
    }
    return this.log[this.log.length - 1];
  }
};

console.log(obj.latest);
// expected output: "c"

If you look at the source code you can see when you call .argv, yargs just calls .parse():

 const yargs = new YargsInstance(processArgs, cwd, parentRequire, _shim);
 // Legacy yargs.argv interface, it's recommended that you use .parse().
 Object.defineProperty(yargs, 'argv', {
     get: () => {
        return yargs.parse();
      },
     enumerable: true,
});