What does the new keyword actually do?

These three lines do quite different things, and the difference comes down to what new adds.

function Person() {} merely declares a function. Nothing runs. By convention the capital letter hints it is meant to be used as a constructor, but JavaScript does not enforce that — it is an ordinary function until someone calls it with new.

const person = Person() calls it as a plain function. No object is created for you, this is undefined in strict mode (or the global object otherwise), and person receives whatever the function returns — undefined if it has no return. This is the classic bug: the code looks like it made an object, then fails later with "cannot read property of undefined".

const person = new Person() invokes it as a constructor. new does four things: creates a fresh empty object, sets its prototype to Person.prototype, calls the function with this bound to that object, and returns it automatically unless the function explicitly returns a different object.

function Person(name) {
  this.name = name;
}

const a = Person("Mai");      // undefined — and `this.name` leaked or threw
const b = new Person("Mai");  // Person { name: "Mai" }

b instanceof Person; // true

Modern code sidesteps the trap by using class, which throws a TypeError if you forget new instead of failing quietly.