Can we cast a generic object to a custom object type in javascript?

Question

For example, I already have this object somewhere in the code, it is a generic object:

var person1={lastName:\"Freeman\",firstName:\"Gordon\"};

Answer 1

No.

But if you're looking to treat your person1 object as if it were a Person, you can call methods on Person's prototype on person1 with call:

Person.prototype.getFullNamePublic = function(){
    return this.lastName + ' ' + this.firstName;
}
Person.prototype.getFullNamePublic.call(person1);

Though this obviously won't work for privileged methods created inside of the Person constructor—like your getFullName method.

Answer 2

This is just a wrap up of Sayan Pal answer in a shorter form, ES5 style :

var Foo = function(){
    this.bar = undefined;
    this.buzz = undefined;
}

var foo = Object.assign(new Foo(),{
    bar: "whatever",
    buzz: "something else"
});

I like it because it is the closest to the very neat object initialisation in .Net:

var foo = new Foo()
{
    bar: "whatever",
    ...

Answer 3

This borrows from a few other answers here but I thought it might help someone. If you define the following function on your custom object, then you have a factory function that you can pass a generic object into and it will return for you an instance of the class.

CustomObject.create = function (obj) {
    var field = new CustomObject();
    for (var prop in obj) {
        if (field.hasOwnProperty(prop)) {
            field[prop] = obj[prop];
        }
    }

    return field;
}

Use like this

var typedObj = CustomObject.create(genericObj);

Answer 4

This is not exactly an answer, rather sharing my findings, and hopefully getting some critical argument for/against it, as specifically I am not aware how efficient it is.

I recently had a need to do this for my project. I did this using Object.assign, more precisely it is done something like this:Object.assign(new Person(...), anObjectLikePerson).

Here is link to my JSFiddle, and also the main part of the code:

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;

  this.getFullName = function() {
    return this.lastName + ' ' + this.firstName;
  }
}

var persons = [{
  lastName: "Freeman",
  firstName: "Gordon"
}, {
  lastName: "Smith",
  firstName: "John"
}];

var stronglyTypedPersons = [];
for (var i = 0; i < persons.length; i++) {
  stronglyTypedPersons.push(Object.assign(new Person("", ""), persons[i]));
}

Answer 5

This worked for me. It's simple for simple objects.

class Person {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  getFullName() {
    return this.lastName + " " + this.firstName;
  }

  static class(obj) {
    return new Person(obj.firstName, obj.lastName);
  }
}

var person1 = {
  lastName: "Freeman",
  firstName: "Gordon"
};

var gordon = Person.class(person1);
console.log(gordon.getFullName());

I was also searching for a simple solution, and this is what I came up with, based on all other answers and my research. Basically, class Person has another constructor, called 'class' which works with a generic object of the same 'format' as Person. I hope this might help somebody as well.

Answer 6

The answer of @PeterOlson may be worked back in the day but it looks like Object.create is changed. I would go for the copy-constructor way like @user166390 said in the comments.
The reason I necromanced this post is because I needed such implementation.

Nowadays we can use Object.assign (credits to @SayanPal solution) & ES6 syntax:

class Person {
  constructor(obj) {
    obj && Object.assign(this, obj);
  }

  getFullName() {
    return `${this.lastName} ${this.firstName}`;
  }
}

Usage:

const newPerson = new Person(person1)
newPerson.getFullName() // -> Freeman Gordon

ES5 answer below

function Person(obj) {
    for(var prop in obj){
        // for safety you can use the hasOwnProperty function
        this[prop] = obj[prop];
    }
}

Usage:

var newPerson = new Person(person1);
console.log(newPerson.getFullName()); // -> Freeman Gordon

Using a shorter version, 1.5 liner:

function Person(){
    if(arguments[0]) for(var prop in arguments[0]) this[prop] = arguments[0][prop];
}

jsfiddle