Writing a method in different ways and others in JS

Apparently there are at least three different ways of writing a method or a function in JS.

// Traditional way
function someVar(one, two) {
  this.f = 'some str';
  // #g = 3;
}

// JS way
const someVar = function (one, two) {
  this.f = 'some str';
  // #g = 3;
};

// Arrow function or lambdas in C#
const someVar = (one, two) => {
  this.f = 'some str';
  // #g = 3;
};

Apparently for OOP, back in the day, JS did not have classes and a method itself was used as a class like below.

function someVar(one, two) {
  this.f = 'some str';
  // #g = 3;
}

const frd = new someVar(1, 6);
console.info("someVar is " + frd.f);

An object can have a property or a method like below.

const newVar = {
  frd: 'some str here',
  sfr() {
    console.info('This is in sfr');
  },
};

One object's methods or properties can be copied or updated to another object aka inheritance or extending the object itself using object's prototype and assign .

function someVar(one, two) {
  this.f = 'some str';
  // #g = 3;

  this.bgt = () => {
    console.info('In bgt now');
    return 1;
  };
}

Object.assign(someVar.prototype, newVar);

newVar.sfr();

console.log('Afre adding ' + new someVar(1, 3).frd);

Suppose one has imported a JS library from unpkg.com. The library does not have a new functionality, it can be easily added or extended this way.