Factory & Constructor Functions

function User(name) {
  this.name = name;
  this.sayHi = function() {
    console.log('Hi, ' + this.name);
  };
}

It doesn't validate the name parameter
It creates a new function for each instance
This constructor function creates a new sayHi function for each instance of User, which is inefficient for memory usage. A better approach would be to put the method on User.prototype so all instances share the same function: User.prototype.sayHi = function() { console.log('Hi, ' + this.name); }. This way, the method is created once and shared across all instances.
It doesn't use arrow functions
It uses string concatenation instead of template literals